New Zealand Prime Minister John Key on Monday said the bill from the devastating Christchurch earthquakes had soared to NZ$25 billion (US$20.2 billion), far more than previously thought.

Earlier official estimates of rebuilding put the costs of the quakes in September and February -- the second of which claimed 181 lives -- at NZ$15 billion.

The prime minister said the costs had risen, without specifying whether that was due to a third powerful tremor that hit New Zealand's second-largest city last week.

"On our estimates, this is the single biggest (proportional) impact of any natural disaster on any developed economy we can find," Key told reporters during a visit to Canberra.

"It is going to cost in the order of eight to nine percent of GDP, round about NZ$25 billion, so it's a very major event."

The quakes were so severe that an estimated 10,000 homes will have to be demolished and entire suburbs abandoned due to liquefaction, where the tremor's shaking turns the ground into unstable silt.

Key said residents were still on edge because of aftershocks that continue to rumble through the area.

"It's had a huge impact on the confidence of the people of Christchurch because there's been approximately 7,500 aftershocks of magnitude 3.0 or above since (the first quake on) September 4," he said.

Key was greeted with a 19-gun salute and a marching band playing Waltzing Matilda as he arrived at Parliament House in the Australian capital Monday.

He inspected the Federation Guard before becoming the first New Zealand leader to address Australia's parliament.

In his address, he thanked Australia, which sent 300 police to help with the recovery effort following the quakes.

"New Zealanders clapped for the Australian presence because it was such a moving and visual demonstration that we weren't on our own," he told parliament.

"Let me tell you, that sense of unity and support mattered more than you might imagine."

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