Prime Minister John Key says two people have died after this morning's 7.5 earthquake.

'On the best information we have there's been two fatalities. At this point we're unable to give you precise information on what caused those fatalities," John in his first address to media since the earthquake.

"We think it's only likely to be two - there are isolated parts of the country that we don't have perfect eyes on so we can't be 100 per cent sure."

'It was most the significant shock I can ever remember in Wellington.”

Kaikoura on the East Coast was the worst affected and an NH90 helicopter will be heading there "very rapidly" to assess the situation.

John says military helicopters are available and once it's practical to do so both he and acting Civil Defence Minister Gerry Brownlee will head to the worst affected areas.

A national emergency hasn't been called at this stage because the 16 regional Civil Defences are all activated and able to "carry out their functions".

The quake was widely felt in both the South and North Islands, as far north as Auckland, causing damage and knocking out power as well as triggering tsunami alerts.

Police earlier said they were trying to access a property north of Christchurch where a person was believed to have died.

The Fire Service was also searching for a person who was believed to be missing after a homestead collapsed in Kaikoura. One person had been found alive in the rubble, reports Stuff.

A 7-person Urban Search and Rescue (USAR) squad was en route to Kaikoura from Christchurch by helicopter.

Assessment teams were travelling to Waiau and Blenheim.

Key was due to fly out to Argentina on Tuesday en route to APEC in Peru.

A decision hasn't been made about whether he will cancel his plans but it's possible the Argentina leg of the trip will be cancelled.