Barfoot & Thompson's managing director, Peter Thompson, and chief executive, Wendy Alexander, confirmed the sacking after conducting an investigation into the leak, which Labour used to show a disproportionate amount of residential sales to people with Chinese surnames.
The staff member admitted passing on confidential sales data, Mr Thompson said.
"The action was taken after careful consideration of all the facts and meeting personally with the member of staff involved."
Mr Thompson refused to release personal details about the staff member or say how the data was leaked, but did say the leaked was over a period of weeks.
"We can offer no insights into the motivation of the staff member concerned," he said.
He said that while the data was passed to media and political figures, it was not given directly to the Labour Party.
Police had not been involved but the company had spoken to its solicitors and the internal investigation was ongoing to establish if anyone else was involved in the leak.
'Extremely disappointing'
Mr Thompson said: "This decision was not taken lightly, but we have very clear and well-defined company policies.
"Our investigation demonstrated conclusively that in this case a line had been crossed and the breach of our policies has provided grounds for dismissal."
He added: "I am extremely disappointed and saddened at the events that have unfolded over the last few days and believe firmly the actions of one person should not detract from the extraordinary commitment and integrity of the Barfoot & Thompson team."
Labour's housing spokesman Phil Twyford, who released leaked real estate figures to the Herald, said Barfoot & Thompson's decision was "extremely disappointing".
In a statement which did not confirm whether the fired staffer was his source, Mr Twyford said: "The whistleblower 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.
"This data provided an important snapshot of what's going on in the Auckland housing market."
He said he had not revealed his source's name to other Labour colleagues or staff, including leader Andrew Little.
"I'm not going to reveal the identity or even speculate about the identity of the person or the firm from which the information was obtained."
He said he believed the leaker should thanked, describing them as "a whistle blower."
"I think the whistle-blower I dealt with did Aucklanders a favour and put this information into the domain out of a sense of public duty. I think Aucklanders owe that person a debt of gratitude."
Under the Protected Disclosures Act, there is protection for "whistle blower" employees but it only applies in limited circumstances.
It applies where an employee reveals serious wrongdoing such as corruption, conduct that poses a risk to health and safety, criminal activities or gross negligence by public officials.
Information sacrosanct
The data released by the Labour Party, and published in the Herald, showed 39.5 per cent of Auckland house sales between February and April were to people with Chinese surnames, despite people of Chinese ethnicity making up just 9 per cent of the 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.
Barfoot & Thompson's chief executive, Ms Alexander, said information held on behalf of clients and customers was "sacrosanct".
"It was entirely appropriate ... that we investigate what data had been shared inappropriately and to whom.
"It was from that general investigation that we have been able to identify that there had been a breach which resulted in the termination of the person. Our investigation needs to be ongoing to ensure the integrity of the data is respected for the people we represent."
She said the saga had been a distraction. There was no evidence that any individuals' private information had been compromised. "It was simply the weekly sales data."
A number of clients had contacted the company this week to say they were "disenchanted and concerned that their data had been compromised".
Ms Alexander said the leaked data had drawn attention to "a component of our business that has received unnecessary attention in my view this week".
"Auckland is second only to Vancouver in being the most culturally diverse city in the world. It's inevitable that a significant level of our inquiry with regard to list and sell is going to come from Chinese purchasers.
"There is nothing unusual, unique or scary about that and we welcome their business."
Mr Thompson reiterated that the investigation was not yet closed.
"If anyone else is caught doing the same thing the same appropriate action will be taken."
Asked whether the level of debate and depth of feeling the leaked information had caused justified the means, Mr Thompson said: "We don't want to get into that debate."
