Auckland's persistently heated property market may force the Reserve Bank to impose yet more restrictions, an economist says.

The average sale price for a home in New Zealand's biggest metropolis has blown out to a new high of more than $750,000.

The latest figures from Barfoot & Thompson, Auckland's largest realtor, show sale prices reached an average of $776,729 in March.

Buyers were not put off by the high prices, with an unprecedented level of trading that set a string of new records.

The agency sold 1597 homes, the highest number ever in a calendar month, a quarter of which had a price tag of over $1 million.

ASB chief economist Nick Tuffley said annual house price growth in Auckland was hitting 15 per cent on some measures, not far off the near 20 per cent ascent in 2013.

Tuffley said the Reserve Bank would be looking to introduce new tools alongside its existing limits on loan-to-value ratios (LVR) to try and cool the market.

The central bank is already looking to crack down on property investors, requiring banks to hold more capital against any loans made to them.

However, Tuffley said further steps might well be required.

"It's not clear whether that'll have enough impact to change the pricing of those loans," he said.

One of the tools likely to be under consideration was a restriction on borrowing based on income to debt-servicing ratios, said Tuffley.

"There's no guarantee they'll come out and do it, but we think they'll look into that sort of measure."

As of October last year, the Bank of England has only allowed 15 per cent of new mortgages to be at multiples higher than 4.5 times a borrower's income.

Applying those rules to an Aucklander borrowing 80 per cent of an average home's value means they would need to be earning over $138,000 to avoid the threshold.

Income caps are favoured by think-tank the New Zealand Institute of Economic Research, because they directly target risky borrowers.

Reserve Bank spokesman Angus Barclay said the bank did not comment on speculation.

However, governor Graeme Wheeler has previously hinted there could be new regulations on the way.

In February, he said the Reserve Bank was worried about the risk of a sharp fall in house prices.

Wheeler specifically raised concerns about prices getting too far out of line with incomes, and said the bank would be talking more about housing in the months to come.

While the Reserve Bank is primarily concerned with the Auckland and Christchurch markets, Tuffley said any new rules would probably be imposed nationwide.

However, limits on debt-to-income ratios would bite the hardest in Auckland, where prices were the furthest out of step with incomes.