UPDATED 5.44pm: The Barfoot and Thompson staff member who leaked real estate data to the Labour Party has been fired.

The real estate company has revealed the results of an investigation into the leak, which Labour used to show the disproportionate amount of residential sales to people with Chinese surnames.

MORE: Labour's housing spokesperson Phil Twyford speaks with Larry Williams

Managing director Peter Thompson and chief executive Wendy Alexander have been briefing media.

The data was leaked to the Labour Party and showed nearly 40 per cent of house sales between February and April were to buyers with Chinese surnames, despite people of Chinese ethnicity making up just 9 per cent of Auckland's population.

Labour says the data suggests offshore non-resident Chinese buyers are aggressively targeting the Auckland housing market, driving up house prices, and have called for restrictions on foreign buyers. But the party has been accused of racism by the Chinese community.

Director Peter Thompson said the action to sack the Barfoot and Thompson staff member involved was taken after careful consideration of all the facts and meeting the staff member.

"Our investigation demonstrated conclusively that in this case a line had been crossed and the breach of our policies have provided grounds for dismissal," Thompson said.

"It is a basic condition of employment for all Barfoot and Thompson staff that information concerning clients and other staff, without exception, is treated with the utmost confidentiality."

Mr Thompson won't release details about the staffer and says he can't offer any insight into their motivation.

"Confidential data was sent by the identified internal source to a number of individuals including the media and political figures. However this does not include direct contact with the Labour party."

Barfoot said earlier this week it was taking legal advice over the leak and if the data came from Barfoot, it had been given illegally.

Meanwhile Labour's housing spokesperson Phil Twyford, who released the data, has already responded to the sacking and is disappointed with Barfoot and Thompson's decision.

Mr Twyford is defending his decision to leak information about Auckland housing sales. He reports he doesn't know if the sacked person was his whistleblower.

"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."

Twyford told Newstalk ZB he doesn't know if the person had the authority to release the information, and he doesn't need to know.

"They put information out there that the government doesn't trust people with. People want to know what's happening in the Auckland housing market."