Tsunami could be 12m high

Northland is seen as being one of the most at-risk regions in New Zealand. Photo / File

A tsunami in Northland is likely to be bigger than previously thought and one generated close to New Zealand could arrive before official warnings or sirens could be activated.

And the one message in a major new national report is "don't wait to be told, just head for the hills".

The 2013 Tsunami Report, commissioned by the Ministry of Civil Defence and Emergency Management and released by GNS Science yesterday, says a tsunami from a local earthquake would arrive in less than hour and one from a regional earthquake in one to three hours, .

People on the coast who feel an earthquake strong enough to make it hard to remain standing, or a weaker earthquake that lasts a minute or more, should get to high ground or go inland as soon as possible.

Along much of Northland's coast, incoming water is likely to be over eight metres high, and over 12m along a significant stretch of the region's east coast.

The good news is that, while the size and impact of a tsunami may be bigger, the actual chances of one sweeping in have not increased.

All New Zealand's coast is now considered at risk from tsunami.

The areas under greatest threat are Northland, part of the Auckland region, Great Barrier Island, Coromandel Peninsula, Bay of Plenty, East Cape, and parts of the Wairarapa coast, Southland, Stewart Island, Fiordland and Westland.

The tsunami hazard review is the first to cover the entire New Zealand coast, and the first since one in 2005.

It calls for emergency departments to emphasise the messages about tsunami with more specific local and regional information.

Civil Defence said this year's report incorporated "significant changes in scientific understanding" since the 2005 report following the Indian Ocean Boxing Day Tsunami.

It focuses on the entire coastline rather than just the main population centres and is intended to reinforce public understanding and boost preparation. The 220-page report found New Zealand had about 10 tsunami of 5m or more since 1840, and that dangerous local and regional tsunami may occur every 40-50 years on average. Northland's tsunami alarm network extends from Mangawhai to Te Hapua.

Night the surge struck

On May 24, 1960, the remnants of a Chilean earthquake which killed about 400 people there sent shockwaves in the form of a tsunami across the Pacific Ocean. It killed 19 people in Hawaii and 85 in In Japan.

It surged on to the Northland coast in the early hours of the morning - fortunately during a low tide - reducing the normal 12-hour tide cycle to about every 20 minutes as the sea sucked in and out.

At Tutukaka, in between the tide surges, people were running on to the sand and getting flounder left stranded by the quickly receding water.

When the big wave hit in the early hours, breakers raged up the narrow-entranced Tutukaka harbour and crashed ashore. At Ngunguru, the tsunami struck and flooded the lower lying area about 5.30am, arriving as a super-high tide but not a wave.

Witnesses saw the water disappear from the Ngunguru basin, then come back in again three or four times over the next few hours.

The tsunami sucked the water out of Whangarei Harbour in a short time, exposing areas that had never been seen before. The same happened at Opua, where there was no huge wave, just a sudden surge in the dead of night.

For much of the next day, the tide ebbed and flowed frequently.

Several times the loaded car ferry was sitting in plenty of water and just a few minutes later stranded on the mud. Witnesses said the harbour looked like a swamp one hour and like a spring high tide the next.