A woman is about to lose her home after a court ruled that a man she claims was a boarder was in fact her de facto partner.

Supermarket checkout operator Linda West, 50, may have to sell her Morrinsville house of 20 years unless she wins an appeal.

The Family Court has ruled the man she claimed was a boarder, John Coll, 55, was her de facto partner of more than three years and is entitled to half the property now they have split up.

They are among many Kiwis being "caught out" for not knowing the rules around legally defined de facto relationships, according to Otago University law professor Nicola Peart.

She is among several lawyers lobbying Parliament for changes to the act to make it more clear and fair. Peart says the Property Relationships Act needs an overhaul.

Justice Minister Simon Power says he is aware of some concerns around relationship property laws and has asked officials to continue monitoring issues surrounding it. Several experts, including Peart, are lobbying for change.

Peart says many Kiwis have lost property or are "at risk" of losing possessions.

"They didn't realise they were under [the Act] so didn't contract out of it," she says.

Contracting out involves the parties agreeing while in the relationship how they will deal with assets if it ends.

According to Statistics NZ there were 8737 divorces last year. There are 700,000 legally married couples, while 379,956 Kiwis are in other relationships including de facto relationships.

According to the Morrinsville judgment, West had separated from her husband in 2002 and shortly after Coll moved in.

She said he was a boarder, while he argued they had a de facto relationship until 2008.

Judge David Brown deemed on a balance of probabilities that a de facto relationship existed.

Evidence that was heard included Coll paying varying rents weekly but for a period he paid nothing.

He said he once took her to a race function, he had given her a ring and she had attended his mother's funeral.

Judge Brown said he was confident there was a sexual relationship. He noted there was no merging of their financial lives. Coll was entitled to a half share of West's home valued at $227,000, less the sum owing on the mortgage.

Coll's car was deemed a chattel to be separated, but not his "hobby" horse stud, which was viewed as separate property.

West says the law "stinks". She will appeal to the High Court at Hamilton this week.

Flaws in the law
Professor Peart's problems with the Property Relationships Act:

* Outdated and amendments in 2001 were "shonky".

* Based on outdated notions of property ownership; people often have assets in trust and no longer "own" them.

* Economic disparity provisions were "a mess" and there were "endless cases" where the courts struggled to decide how to compensate a spouse who had stayed at home and not been able to build their career.

* Cases were costly and could take a long time to be determined.