Tsunami threat passes but what is Whanganui's future risk?

Tsunami warning posters are scattered around Whanganui's coastline. Photo / Bevan Conley

Thursday's tsunami warning for the Pacific Ocean is a timely reminder to stay ready and be prepared to evacuate, Whanganui Civil Defence manager Tim Crowe says.

Authorities issued a tsunami advisory for coastal parts of New Zealand following a 7.7 earthquake near New Caledonia at 2.20am on Wednesday.

The tsunami advisory predominantly related to Northland but Whanganui was to expect strong and unusual currents.

By 9am on Thursday the advisory was cancelled as the threat had passed.

But Crowe said all coastal areas in New Zealand had to be prepared for tsunami.

"We are one of the lower risk parts of the country because of geography," Crowe said.

"But a tsunami from a source like we had this morning has the potential to come the middle of the Tasman Sea and can run against our coastline.

"Tsunami do travel across the ocean. For example, travel times from a risk from the Cascadia region past Seattle, is about 14 and a half hours.

"They travel in the deep ocean at the same speed as a jumbo jet."

He said the most common threats or events to New Zealand come from South America.

"They regularly cause small size tsunamis such as Northland, East Coast. South America isn't a huge risk to us."

The biggest threat locally is the Hikurangi Trench, which is just off the lower east coast of the North Island.

"That's an earthquake no one will sleep through, even here in Whanganui, when that goes.

"That has the potential to push a tsunami wave or pulse through the Cook Strait and head up our coastline. That's a lot quicker, that might be an hour."

Crowe said the Civil Defence wasn't called 'Doom and Gloom' for nothing.

Whanganui Civil Defence Manager Tim Crowe said Whanganui is pretty well protected geograpically thanks to Taranaki. Photo / Stuart Munro

"We look at it as it might not happen in our lifetime, but the best thing you can do for people is get them out of the way.

"Being aware these earthquakes of a certain type that can cause a tsunami aren't uncommon. We get alerts on a regular basis, not national but from some of the other sites we monitor."

Historically, tsunamis have hit the Whanganui coast, albeit small.

"You might see tidal bores. Where you get small pressure waves up the river."

Crowe said having the Whanganui River through the town doesn't make a huge difference to the response or impact.

"Tsunami is a harbour wave, and whilst we have a harbour, you have things like the South Beach dunes which give it a great deal of protection. It is a narrow entry area.

"It's highly unlikely that it would breach the banks."

Whanganui has a three-tiered evacuation procedure Whanganui had with numerous boards around beaches and the river.

"They really show you where to get where you're safe," Crowe said.

"Public education is a huge part. The message we have is if it's long or strong, don't wait, get gone. If there is something that happens close to us, there isn't a lot of time to get warnings out.

"The key message is if we have an earthquake that lasts longer for a minute and you are struggling to stand up and you are near the coast, don't wait for people to tell you what to do. Move inland or to higher ground.

"If there are ones like we have early Wednesday morning, find out what's going on and if there is a need to evacuate. We are certainly here to update people and push the information out."'