Reserve Bank of New Zealand governor Graeme Wheeler has agreed with MPs on parliament's finance and expenditure committee that new restrictions on property investors could make it easier for foreigners to buy houses in Auckland.

The bank announced on Wednesday that from October 1 any mortgage secured on a home in Auckland not occupied by the owner would need a 30 per cent deposit.

The move is intended to help cool down the city's overheated housing market by making it more difficult for speculators to buy properties.

Mr Wheeler appeared before the committee following the announcement, and Labour MPs asked him what effect the restrictions would have on non-resident cash buyers.

He said they wouldn't be affected by the restrictions, unless they applied for a mortgage with a New Zealand bank.

"To the extent that there's less aggressive competition from investors, that gap could be filled by various parties," Mr Wheeler said.

"Hopefully we will see more first home buyers get access to the market."

Mr Wheeler agreed with Labour's Grant Robertson that it would be useful to have data on the number of non-residents who are buying houses in Auckland.

Opposition parties have been calling for the data to be collected but the government doesn't think it's necessary.

"It would be helpful to have the data," Mr Wheeler said.

"Those buyers, of course, include New Zealanders living abroad and people who may intend to come here to live."

The New Zealand dollar weakened after Mr Wheeler reiterated to the committee he still considered the currency's strength "unjustified and unsustainable", despite its five per cent decline on a trade-weighted basis over the last three weeks.

The kiwi fell as low as 73.61 US cents from 74.03 cents before his comments.