The New Zealand Reserve Bank has signalled it could introduce further restrictions on investor borrowing in the housing market by the end of the year.

Deputy governor Grant Spencer says the bank is concerned about financial stability and is closely considering measures to contain what it sees as a growing risk caused by high house prices and increased borrowing.

"The Reserve Bank has a range of policy options available," Mr Spencer said in a speech on Thursday.

"One is tighter LVRs to counter the growing influence of investor demand in Auckland and other regions."

He said one approach would be to adopt a single national LVR limit on investors.

"Given that the banks have much of the relevant systems work in place, we expect that such a measure could potentially be introduced by the end of the year."

In November last year, the bank restricted investor borrowing in Auckland to 70 per cent of the value of the property they were buying.

In the rest of the country the limit is 80 per cent.

He said the bank was also looking at a debt-to-income "speed limit" on mortgages.

Banks would be consulted before any decisions were made.

Finance Minister Bill English has previously confirmed that he was discussing debt-to-income restrictions but said decisions were still some way off.

Mr Spencer ruled out increasing interest rates as a means of curbing mortgage lending because that could drive inflation below the bank's target range of between one and three per cent.

"The global environment is likely to keep interest rates low for some time yet," he said.

Explaining the bank's concerns, Mr Spencer said a severe housing correction - meaning a significant fall in house prices - could pose "real risks" for financial system stability and the economy.

"The banks are heavily exposed to housing with mortgages making up around 55 per cent of total assets," he said.

"Household debt, at 163 per cent of household income, is at a record level."

On Tuesday Prime Minister John Key suggested that if the bank intended further investor restrictions it should "get on with it".

That was after Quotable Value reported steep increases in house values across the country, and particularly in Auckland.

It said the prospect of further restrictions seemed to have prompted a surge in investor activity, with buyers acquiring as many properties as possible before the rules changed.