Reserve Bank deputy governor Grant Spencer will give a speech on housing tomorrow and it is expected he could signal increased LVRs.
Property Institute of New Zealand head Ashley Church predicted the threshold for investors in Auckland would be raised to 40 per cent before the end of the year.
The new QV figures showed the average Auckland house value was now $975,087. An investor would require a deposit of at least $390,000 if a 40 per cent LVR limit was introduced.
The higher threshold would not have a big impact on investors with multiple properties, Mr Church said.
"Anybody that has got a portfolio of more than a couple of properties, they're probably going to be going up by that amount anyway."
Mr Key said the Reserve Bank had given him no signals about whether it would tighten lending restrictions.
"But my sense is potentially one of the risks is you have got people buying rental properties at the moment, borrowing more money but fearful that the Reserve Bank is going to move," he said.
"If they are going to make changes, probably they should just get on with it."
Asked whether restrictions should focus only on Auckland, Mr Key said they should apply to all "high growth" areas.
That list also includes Hamilton, Tauranga, Christchurch and Queenstown.
Labour's finance spokesman, Grant Robertson, said yesterday that there was scope for higher LVRs, but only if they targeted investors.
"What we don't want is the comprehensive approach to LVRs that would force first-home buyers out.
"But we think there is room for this."
Mr Robertson said he was not surprised by Mr Key's comments.
"Once again he wants to rely on the Reserve Bank governor to get him through a tight spot."
In June the Reserve Bank left the official cash rate at 2.25 per cent, with governor Graeme Wheeler saying investors could soon be targeted by new LVR lending rules.
The Reserve Bank introduced LVRs of 20 per cent in 2013 to rein in the housing market, and in 2015 it raised the limits to 30 per cent for investors in Auckland.
According to the new QV figures, national house prices shot up 5.6 per cent in the past three months, the fastest increase in 12 years, driven by high numbers of migrants, low interest rates and a rush of landlords.
The national average value is now $590,909.
Auckland house values shot up 4.7 per cent in the past three months and 16.1 per cent since June last year, reaching a new average of $975,087.
The Queenstown Lakes District had the greatest quarterly increase, up 10.7 per cent during the past three months and 25 per cent year on year.
Hamilton values rose 29 per cent since June 2015 and 6.9 per cent in the past three months to hit an average $492,403. Tauranga values rose 23.6 per cent in the past year and 4.9 per cent in the last quarter, showing an average value of $599,915.
Latest QV figures
• $975,087 - Auckland: Average house price, up 4.7% in past three months and 16.1% since June last year
• $492,403 - Hamilton: Average house price, up 6.9% in past three months and 29% since June last year
• $599,915 - Tauranga: Average house price, up 4.9% in past three months and 23.6% since June last year