A fair split is drafted, not assumed.

The Property (Relationships) Act 1976 sets the framework: once a de facto relationship, marriage or civil union has lasted three years, the property built up during it is presumed to be shared equally on separation. Whose name sits on the title, and who paid, generally does not decide the outcome — the home and its contents, joint accounts and savings, vehicles, businesses and the KiwiSaver contributed while you were together all fall into the pool to be divided.

The real contest is usually over what stays out. Separate property — what each partner owned before the relationship, along with inheritances, gifts and ACC compensation payments — normally remains with its owner, but only while it is genuinely kept apart. An inheritance paid into the mortgage on the family home, or pre-relationship savings mixed through a joint account, can be absorbed into the pool through intermingling.

Equal sharing is a default, and the Act allows departures from it: relationships under three years are divided on contributions instead; a partner left significantly worse off financially can seek an economic disparity adjustment; and under s13, extraordinary circumstances that would make equal sharing repugnant to justice can move the division off 50/50 — a deliberately high bar the courts reserve for the rare case. Working out which side of those lines your assets fall — early, on the documents — is most of the job.

In New Zealand nothing formal has to be filed to separate — the law asks whether the relationship has genuinely ended in substance, with the couple leading separate lives even if still under the same roof. A separation agreement puts that on paper: it fixes the date the relationship ended, which is the date the two-year divorce clock and other time limits run from, and records what has been agreed about property, debts, the children and any support.

For the property side of the agreement to bind, section 21F of the Property (Relationships) Act 1976 imposes non-negotiable formalities: the agreement must be written and signed, each partner needs advice from their own lawyer first, that lawyer must certify the advice on the document, and no one can have been pressured into it. An agreement signed around those requirements — usually to save fees — is the kind the Family Court later sets aside, and re-opening a division costs far more than doing it correctly the first time.

Two things routinely get missed. KiwiSaver is one: the contributions that went in while you were together are relationship property, however much they feel like personal savings, and leaving them out of the agreement leaves them unresolved. The other is the parenting side — the Family Court always keeps oversight where a child's welfare is in issue, so parenting clauses work best drafted with room to move as children grow, not as fixed terms.

A contracting out agreement puts the division rules in the couple's own hands — under section 21 of the Property (Relationships) Act, partners can settle between themselves how their property would be dealt with, so the default equal-sharing regime never has to be argued over. The label "prenup" undersells the tool: the same agreement can be signed before the relationship, years into it, or as part of untangling it at the end — and it earns its keep most where one partner arrives with more to protect, whether an established business, family money or an inheritance, a home already owned, or children whose interests come first.

The formalities mirror the separation agreement rules, and they are where these agreements live or die: each partner needs their own lawyer's advice, and the agreement must carry each lawyer's certificate confirming that the advice took place. Skip or shortcut that step and the agreement is exposed — certification failures are the most common route by which contracting out agreements come apart in court.

None of this signals mistrust; the point is certainty, settled while both partners are on the same side of the table. The wider case law is worth knowing too — the PRA reaches beyond separations into estates and family protection claims, and our summary of the leading authorities in that territory, from Public Trust v Whyman through to Cartwright v Joseph, is linked below.