Prime Minister John Key has defended the cost of housing in New Zealand, saying under his Government house prices had gone up only 43 per cent.

In response to Labour housing spokesperson Phil Twyford's calls for the Government to "declare a state of emergency", Key defended his Government's actions.

Listen: John Key speaks to Newstalk ZB's Mike Hosking housing affordability:

"National house prices under the 9 years Helen was Prime Minister went up 102 per cent, under us they went up 43 per cent," he said.

"Auckland house prices under them and under us, broadly in Census terms, went up the same. If it's a national state of emergency now, in a crisis now, why wasn't it when Helen was Prime Minister?"

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Key told NewstalkZB's Mike Hosking the cost of an average Auckland house is nine times that of the average earnings.

"To give you an example Sydney is 12 times and we're about the same with Melbourne," he said.

"If you want to resolve this issue then absolutely build more houses and there is no question that this is not happening - we're in the middle of a building boom."

Social housing Minister Paula Bennett said Government was doing everything it could.

Listen: Phil Goff speaks to Early Edition's Rachel Smalley about housing:

"I don't think Phil Twyford is engaging his brain, he's just come out with 'state of an emergency' without actually thinking about what that would mean."

Auckland mayor hopeful Phil Goff launched his campaign with a housing policy aimed at slowing down immigration.

Our transport and housing provisions are simply not coping with that level of growth as most of that growth is coming in Auckland.

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"Our transport and housing provisions are simply not coping with that level of growth as most of that growth is coming in Auckland," he said.

"My challenge to Government or any Government is that either you've got to slow down the rate of migration or provide the funding that lets us put that infrastructure in place."

- with additional reporting from Newstalk ZB.