A staff member at Auckland's largest real estate company has been fired for leaking sales data which ended up in the hands of the Labour Party.

The sacking has added fuel to the political row that's raging around the impact offshore Chinese investors are having on Auckland's overheated housing market.

Labour's Phil Twyford released the figures at the weekend, saying they showed 40 per cent of houses sold in Auckland between February and April went to people with Chinese names.

The government accused Labour of "playing the race card" and at least one party member is reported to have resigned in protest at its tactics.

A Chinese language newspaper published in Auckland has accused Labour of irresponsible racism.

Party leader Andrew Little is standing by the decision to release the information and he's calling on the government to set up a register of foreign property buyers.

However, 3News reports he's pulled out of an agreement to speak at the Association of Migration and Investment's annual conference, citing conflicting commitments.

The real estate company, Barfoot & Thompson, investigated after suspecting its data had been leaked, managing director Peter Thompson said at a media conference on Wednesday.

The investigation found the person has made multiple leaks over a number of months to media and political figures, but there was no evidence yet of the data being given to Labour directly.

"While our investigation is still ongoing, we are able to confirm conclusively that confidential data was sent by the identified internal source to a number of individuals," Mr Thompson said.

He said he wouldn't be identifying the staffer and didn't know the person's motivations behind the release.

Chief executive Wendy Alexander said the integrity of the data the company held was "sacrosanct".

"A number of our clients and customers ... have contacted us this week to say that they are disenchanted and concerned that their data could be compromised. And it goes from just the weekly sales being compromised to what else might be shared."

Ms Alexander also said there was "nothing unusual, unique or scary" about many buyers coming from China: "We welcome their business."

Mr Twyford says the sacking is "extremely disappointing".

"The whistleblower who I worked with wanted to shine a light on what is a very real issue for New Zealand - foreign investment pushing up house prices and shutting people who live here out of the property market," he said.

"The data provided an important snapshot of what's going on in the Auckland housing market."