The number of people getting IRD numbers for buying and selling property is about 300 per month.

Auckland house prices are set to "take off" again as Inland Revenue clears a backlog of IRD number applications from cashed up overseas buyers, the Property Institute says.

About 300 foreigners a month are being issued IRD numbers for the purpose of buying or selling property, figures released by Inland Revenue show.

Under legislation introduced in October, overseas, non-resident individuals buying or selling property in New Zealand must provide their IRD number and have a New Zealand bank account.

The move was designed to improve Inland Revenue's ability to effectively collect tax on property transactions.

It was also seen as the first real attempt to get an idea of just how many foreigners were investing in New Zealand property, particularly in Auckland where overseas buyers were being blamed for soaring house prices despite no supporting data.

At the same time a new bright-line-test was introduced, meaning property owners pay tax on capital gains earned on a house sold within two years of purchase. It does not apply to the family home.

An IRD number is a unique number used by Inland Revenue, banks and financial institutions, government departments and employers to identify individuals for tax purposes.

In the three months to January 31 Inland Revenue issued 1025 IRD numbers to foreigners who applied for one for the purpose of buying or selling New Zealand property.

In the same period 20,409 houses were sold in New Zealand, 6265 of which were in Auckland.

If each new IRD number holder bought one house in Auckland they would account for 16 per cent of total sales during that period.

For the whole of New Zealand they would account for just 5 per cent.

Before the legislation came into effect IRD application forms did not ask the reason for applying for an IRD number.

Now applicants have the choice of selecting as many as six reasons including for employment, applying for a student loan and buying or selling property.

A total of 37,637 IRD numbers were issued to foreigners for all reasons from October to February.

No applications were processed for overseas buyers in October.

Property Institute chief executive Ashley Church said legislation changes had slowed Auckland's housing market but he did not expect it to last.

The figures suggested that a backlog of IRD applications was being cleared, he said.

Towards the middle of the year property prices would "take off" as wealthy overseas buyers were issued IRD numbers, he said.

"Those people will progressively reenter the market and they are people who are cashed up and in a strong position to pay over the odds for property," Church said.

It was not the only factor that would fuel continued price rises in Auckland but it was an "exasperating factor", he said.

Land Information New Zealand is collecting IRD information on property sales and would release data in April.

Independent economist Shamubeel Eaqub​ said information on buyers and sellers' nationality, country of residence and the location and number of homes being purchased would be very helpful.

Most foreign buyers were going to be in Auckland, he said, but they had not been the main driver of rising house prices.

"Are they a contributing factor, absolutely, but the main factor, absolutely not," Eaqub said.

Nick Goodall, senior research analyst at property information and analytics company CoreLogic​, said it was difficult to analyse the data because there was no historical data to compare it against.

But on the surface foreign buyers appeared to be a small part of the market.

"It's quite insignificant and nothing like the numbers that had been reported based on some of that name analysis," Goodall said.

In July the foreign buyers debate reached a climax when Labour used surnames from electoral roll data to suggest buyers of Chinese descent accounted for 39.5 per cent of sales in Auckland.

Goodall expected the rate of IRD numbers issued to remain consistent.

New Zealand Property Investors Federation executive member Andrew King said the IRD numbers issued to foreigners appeared to be low levels but it was not known how many properties individuals purchased.

Auckland's housing market had cooled in recent months in part due to bright-line-test legislation but more because there were less Chinese buyers due to restrictions on moving cash out of China and rental yields had not kept pace with rising house prices, he said.

"The yields in Auckland have been falling," King said.

Investors were either looking to other areas for better yields or choosing not to buy, he said.