It's a case of when, not if, a major tsunami hits the Hawke's Bay coast.

Depending on what time of day it hits, more than 15,000 might not make it to high ground in time.

It will likely be preceded by a major earthquake at the Hikurangi Trench close offshore, giving little time to evacuate. Estimates have ranged from 12 to 50 minutes, but even on the higher range thousands will be unable to make it to high ground in time.

"So far, based on the modelling, around about 15,000 would have difficulty in actually getting out of the evacuation zone in 20 minutes," GNS Science tsunami modeller William Power said.

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"A typical time, for a tsunami after an earthquake on the Hikurangi trench getting to Napier, is in the 30- to 50-minutes range, probably mostly around 40 minutes.

"So you can see that there is a problem that not everybody is going to easily get to high ground in time."

A worst-case tsunami would involve a surge of water 13m high. Just over half of those in the tsunami zone would make it to safety in the 20 to 40 minutes before the biggest surge hits.

Napier lacks high ground, with Napier Hill one of the only options, making it an obvious destination for the students and teachers of Te Awa School.

Te Awa School Principal Tim Van Zyl said in a worst-case scenario not all students and teachers might make it to Napier Hill in time.

"My fastest ... does it in 12 minutes and that is running as fast as he can. My slower students - my 5-year-olds - take around 25 to 30 minutes.

"Our plan is to travel down Georges Drive up the Lucknow stairs.

"We know from what happened in Japan in the 2011 tsunami that a lot of people or parents perished coming towards the school, trying to come and get their children.

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"So it is really important that parents know that we are going to be moving that way and that they are to go up towards the Hill and meet us up there."

Adding to the dangers of broken roads and fallen power lines is liquefaction, turning the school's run to high ground a wade through mud.

With the possibility of 20,000 people squeezing onto the narrow streets of Napier Hill, leaving as many to be swept away, the Government is looking at developing building codes for special structures, which are common in Japan, for what is called vertical evacuation of people.

"The Ministry of Business, Innovation and Employment is currently developing New Zealand building guidance for vertical evacuation," Hawke's Bay Civil Defence Emergency Management Group Manager Ian Macdonald said.

"Japan has structures specifically designed for vertical evacuations, and the US has building standards for it, including foundations that are very deep and reinforced to a higher standard than most buildings, and an open ground-floor level to allow water to flow through.

"While we advise everyone to leave evacuation zones and follow their route to a safe location, we understand people may need to consider other options if they're not sure they'll be able to get out of the tsunami evacuation zone in time.

"If there is a multi-level, sturdy building on your evacuation route that you can access, you could choose to go up – look for reinforced concrete or structural steel buildings.

"However, the building may not be built to withstand the impact of a tsunami. If you choose to evacuate vertically, you should go to at least the third floor. If your building doesn't have a third floor, staying there is not a good option.

"Our message for tsunami remains – If you feel an earthquake that is either longer than a minute or strong enough that it is hard to stand up, then get to high ground, out of all zones, as soon as the shaking stops. If it's long or strong, get gone."

He said people needed to know if they are in a tsunami evacuation zone and, if they are, practise their tsunami evacuation route with their family.

A 13m surge could flood all the way to Whakatu, and while it is an unlikely event in people's lifetimes, a smaller surge still poses a threat to thousands of lives.

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