A husband and wife from one of New Zealand's wealthiest philanthropic families are taking their former son-in-law to court to recoup half of the $422,000 they loaned for the purchase of a home.

Bruce and Marion Goodfellow have launched legal proceedings against Nigel Brown after his split from their daughter Claire.

And lawyers and real estate experts say such "bank of mum and dad" disputes are becoming increasingly common.

Specialist divorce lawyer Jeremy Sutton said around 25 per cent of cases he handled dealt with these issues. "Everyone needs to be aware of the complications if they divorce."

Parents who gift their children money to buy their first homes are being urged to seek a "pre-nup" after a rise in the number of divorces ending in legal action.

According to court documents, the Goodfellows say they loaned $422,000 to the couple so they could buy a house on Auckland's North Shore.

But Brown said the money belonged to his ex-wife, and he shouldn't have to pay it back.

Brown said the money was part repayment of a debt owed to her by a company called Amalgamated Dairies, which is headed by Bruce Goodfellow, head of the Rich-lister family and worth an estimated $500 million. National Party president Peter Goodfellow is his brother.

In a statement from his lawyer, Brown said it was a sensitive matter and he did not wish to comment.

The Goodfellows declined to comment.

Property records show Nigel Brown and his former wife purchased 88 Chelsea View Drive in May 2005. The new owner bought the home last year for around $830,000.

The couple also formerly owned 8 Zion Rd which has a council valuation of $810,000.

The matter is set down for a three-day trial in August.

Sutton said parents who offered help with their children's first home purchase should document clearly whether it was a gift or a loan.

"To prove it is a loan or gift is expensive and may require a full court hearing and huge legal costs and expenses," he said.

"It is very hard for people in their 20s and 30s to fund a house deposit in Auckland. Therefore they look to their parents for help.

"People want to put the kids up the property ladder and the only way, often, is to give them a sizeable amount, whether it's a loan or a gift. In this country we're not that good at formalising things. People need to be aware when they give a sum of money to know on what basis that is given."

Sutton said the parental debt was even more common among Asian, and particularly Chinese homebuyers, who would often receive six-figure sums from their parents.

"Just been in a case and another one coming up. It's very challenging. There's often not a lot of written evidence."

Lawyer Chris Patterson said he had been involved with similar cases where a divorced couple disagreed on whether a loan from the wife's parents should be included in relationship property debt.

"Where you've got a couple who are struggling to get into the market, if they're going to get a helping hand from one or more members of their family, they need to document the basis on what the money is being advanced to them.

"That deal needs to be clearly understood by both members in the relationship."

Patterson said the money would be regarded as a loan even if only one partner was aware that it was.