Are Prenups Legal in Australia?

Someone has told you prenups are not worth the paper they are written on. A neighbour, a mate at work, maybe the person you are about to marry.

They are wrong. The short answer is that prenups are legally binding in Australia. Lawyers don’t really use the word prenup here, or prenuptial agreement. The legal name is a binding financial agreement, or BFA. They are legally enforceable if they are done properly, and that is the key.

What a prenup can set out is property, financial resources, and spousal maintenance (s 90B, Family Law Act 1975). What it cannot plan for is care of your children. Parenting matters and child support sit outside a financial agreement, so those need a different document. The clause-level detail is in what a prenup usually covers.

Already separated? You can finalise things with a BFA, or with consent orders, which can cover both parenting and property matters.

What makes a prenup binding?

The agreement has to be in writing. Five more things have to be done, and they come from s 90G of the Family Law Act:

• It is signed by both of you.
• Each of you received independent legal advice before signing, about the effect the agreement has on your rights and about the advantages and disadvantages of signing it.
• Each lawyer gave a signed statement confirming that advice was given.
• A copy of that statement was handed to the other party or their lawyer.
• The agreement has not been terminated or set aside by a court.

Independent means what it says: two solicitors from two different firms, and you cannot share one to save money. The Federal Circuit and Family Court of Australia sets out the same requirements.

The step people skip is the fourth one. The certificates get signed, and then nobody swaps them.

People hear the word binding and assume it means untouchable. No contract works that way:

Sure, you can make any contract in Australia, and if a party is under duress whilst they sign that contract, is that contract going to be binding? No, it’s not. And the same thing applies here to prenups. There are strict rules under the Family Law Act to make sure a binding financial agreement holds up.

What if the paperwork was not done properly?

A defect is not automatically fatal. This is the part people don’t really appreciate.

Under s 90G(1A), a court can declare an agreement binding even where the advice or certificate requirements were not met, if it would be unjust and inequitable for the agreement not to bind. So “their lawyer stuffed up the certificate” is not the escape hatch people assume.

When can a court set a prenup aside?

The list is defined and closed. For married couples it sits in s 90K, and for de facto couples in s 90UM. A court may set an agreement aside where:

• it was obtained by fraud, which includes not disclosing a material matter
• it is void, voidable or unenforceable
• circumstances since have made it impracticable to carry out
• since you signed, a material change has happened relating to the care, welfare and development of a child, and the child, or a party who has caring responsibility for that child, will suffer hardship if the agreement stands
• a party behaved unconscionably in making it
• it was entered into to defraud or defeat a creditor
• it was entered into to defeat the interests of another de facto partner
• the agreement covers superannuation that cannot be split, or a hold on a super payment that is not going to be lifted (s 90K(1)(f) and (g))

Regret is not on that list. Neither is an agreement that simply turned out badly.

Disclosure is the ground that decides most of these

Non-disclosure of a material matter counts as fraud under s 90K(1)(a). And since 10 June 2025 the duty of full and frank disclosure sits in the Act itself, at s 71B for married couples and s 90RI for de facto couples, and it applies expressly to a s 90K application.

In plain terms: hide an asset to make the deal look better and you have handed the other side the strongest ground there is.

The case these are still measured against

Thorne v Kennedy [2017] HCA 49 is the one every family lawyer in the country knows. He held assets worth more than $18 million. She had almost none. Eleven days before the wedding he told her it was off unless she signed. Her own solicitor advised her not to. She signed four days before the wedding.

The High Court unanimously set both agreements aside for unconscionable conduct, and a majority also found undue influence. The judgment summary has the detail.

The lesson is about timing and pressure, not paperwork. She had her own advice and a signed certificate. It made no difference. Give your partner months to consider it, instead of dropping it a few days before a big event.

The matter where our advice was not to sign

A client came to us wanting a prenup. We told them honestly it was a terrible idea.

They had been together more than 20 years, they had three children, and their assets were completely intertwined. Then one of them refused to disclose or value their business, and there was a disagreement about what it was even worth.

So without a formal valuation, we had a gaping hole in the prenup. That’s why in this case, we actually told our client, don’t do it. Signing a prenup with dodgy disclosure was a recipe for disaster. Now, prenups can protect you, but only if you play with all your cards on the table.

That is the practitioner’s test: not whether the file is tidy, but whether both of you genuinely had all the numbers in front of you when you signed.

How much does a prenup cost in Australia?

You both need your own lawyer, so a properly drafted, straightforward agreement generally runs between $5,000 and $10,000 for the exercise, with the two of you separately advised.

That cost is the real reason people skip it, and I understand why. Here’s the catch: arguing over the same money after a separation costs multiples of that. If the number is the sticking point, there are other ways to protect assets worth looking at first.

Do prenups apply to de facto couples?

Yes. You do not have to be married. An agreement can be made before a de facto relationship starts (s 90UB), during it (s 90UC), or after it ends (s 90UD).

The binding requirements are in s 90UJ and the set-aside grounds in s 90UM. Same structure, different sections.

Does a prenup expire after 10 years?

No. There is no fixed expiry date, and the ten-year figure is not a rule anywhere in the Act.

Two things can end one. A sunset clause switches it off on the date you nominated, or a court sets it aside on one of the grounds above.

Time still matters, just not the way people think. In practice a long-standing agreement is harder to unpick, and what ages badly is the content, not the document. Get yours reviewed down the track, after a child arrives, after you buy or sell property, or after a business changes shape.

The honest pros and cons

The case for one is control. You decide what happens to your money instead of handing that decision to a court. You protect what you brought in, and you can cover money that has not arrived yet, like an inheritance or a business you are still building.

One agreement we drafted kept a house in the owner’s name, recorded every contribution his partner made towards it, and set out that she would be repaid those contributions plus a bit extra if they separated. Decided in advance, in writing, by them.

Let’s be honest about the case against. It is not cheap, it is an awkward conversation, and plenty of couples avoid it for that reason alone. It is not bulletproof, because a court can still set it aside, and it does nothing at all for parenting.

Is an overseas prenup valid in Australia?

Usually not, on its own. A document only becomes a financial agreement under the Act if it is expressed to be made under s 90B, s 90C or s 90D (or s 90UB to s 90UD if you are de facto). An overseas prenup will not say that, because it was drafted under someone else’s law.

If you signed one offshore and you are now living here, you generally need a fresh Australian agreement.

What to do next

Talk to your partner first, before a lawyer and before a draft. An agreement that arrives as a surprise is the one that gets challenged, and you have just read what that looks like.

Then each of you gets your own lawyer. That is not us being difficult, it is s 90G.

If you want to work out whether one is worth it in your situation, have a free ten minute call with me and my team, no obligation. We will tell you your options, what it would cost, and whether you actually need one. Our prenup lawyers do this every week. Call 1300 614 732 or send us a message.