UP AND UP: New Zealand's now owe more than $200 billion on their mortgages.

Kiwis' mortgage debt has officially climbed past $200 billion, a mountain of borrowing which has doubled in the space of 10 years.

But while experts are warning homeowners to remain cautious of the overheated property market, they say the blowout is no cause for immediate concern.

The latest Reserve Bank figures show mortgage debt topped $200b for the first time in January, up from just over $100b at the same time in 2005.

As a proportion of gross domestic product, the debt burden has increased from roughly 69 per cent to 88 per cent.

Squirrel mortgage broker John Bolton said the $200b debt was "a hell of a scary number" which demonstrated Kiwis were up to their eyeballs in debt.

The average mortgage for first-home buyers in the heated Auckland market was now close to half a million, he said.

QV figures released this week showed average Auckland properties were now selling for $786,106, with only three suburbs across the whole city squeaking beneath the $500,000 mark.

However, Bolton said banks' annual mortgage lending growth had slowed to a relatively modest 3 to 4 per cent in recent years.

That was in dramatic contrast to the boom before the global financial crisis, when lending growth ran hot at 12 to 15 per cent.

New Zealand Institute of Economic Research economist Shamubeel Eaqub said the prices of millions of properties outside the major hotspots were still below the previous peak in 2007.

Homeowners most at risk to a change in economic conditions were those who had taken on a large debt in recent years, he said.

"If house prices fall and they lose their jobs, that would be the nightmare scenario where you've got both a need to sell, and an inability to service the debt," Eaqub said.

"But we haven't seen that in the past, and we don't think this is the year it's going to happen."

Eaqub pointed to low interest rates, a record influx of immigrants and intense mortgage competition as factors supporting house prices.

However, he said prices in markets like Auckland could prove "unsustainably high" if and when those factors changed.

Infometrics senior economist Matt Nolan said the burden of mortgage debt was not spread equally among households.

"A sharp drop in prices would put a specific group underwater," he said.

"However, unlike the US, when that happens people can't just send their keys back to the bank."

The impact on broader financial stability would depend on to what extent that group defaulted, and how much that increased the chance a bank would fail, Nolan said.

Bolton said looking at Auckland in isolation, there was nothing to suggest house prices would stop their ascent.

However, New Zealand was highly influenced by broader global economic factors and events.

"People are completely oblivious of what's going on," Bolton said. "If you overlay what's going on around the rest of the world, all bets are off."

Bolton said he had stressed to his clients that they should not take any unnecessary risks.

"By all means take measured risks, but don't be silly in this market," he said.

"Some of the sensibleness is starting to wear a bit thin."

Reserve Bank governor Graeme Wheeler has signalled the central bank will be talking more about the housing market in the next few months.

Yesterday the bank announced it was consulting on new rules for residential property investors, expected to increase the cost for landlords and improve financial stability.