A tsunami would put the Christchurch seaside village under more than 2.5m of water.

Most of Sumner will be under more than 2.5 metres of water if a large tsunami hits at high tide, new computer modelling suggests.

As part of an assessment of the coastal hazards facing Christchurch and Banks Peninsula, the city council has been looking at would happen if a tsunami generated by a large earthquake in South America was to hit the Canterbury coastline.

According to computer modelling done by Niwa, Sumner and pockets of Redcliffs, Taylors Mistake and South New Brighton would be hit the hardest and potentially be under more than 2.5 metres of water if the tsunami arrived during the high tide.

The impact would be much less severe if the tsunami was to hit during the low tide.

"Whether it (the tsunami) arrives on a low tide or a high tide makes a big difference to the impact on our coast," said Christchurch City Council natural environment and heritage unit manager Helen Beaumont.

Tsunami events that affect the Canterbury coastline are rare but they do occur.

In February 2010, a magnitude-8.8 earthquake off the coast of Chile triggered tsunami waves that arrived along the Canterbury coast after about 14 hours. Little damage was done but water over-topped the fisherman's wharf at the Lyttelton Port and the Governors Bay jetty. Some low lying areas of Teddington and Charteris Bay also flooded.

The last tsunami to significantly affect the Canterbury coastline was in 1960 and was generated by a magnitude-9.5 earthquake off the coast of Chile. It caused fluctuations of up to 4.5 metres above normal sea level along the coast but its impact was minimal as it arrived at low tide.

Beaumont said distant source tsunamis – generated by earthquakes in South America – posed the biggest flooding threat to Christchurch and Banks Peninsula but usually came with plenty of warning as they took 10 to 12 hours to arrive.

Although the effects of such a tsunami could be devastating people should have time to evacuate, she said.

Because the likelihood of a major tsunami hitting the Canterbury coastline was very low, the council was not proposing to address the risk through planning rule changes. It would deal with it through Civil Defence.

"Its all about what's the community's appetite for risk. The whole coastline of New Zealand is settled despite the tsunami risk and I don't think any of us are going to turn away from it," Beaumont said.

Sumner resident Susan Kaschula whose house would be among those under water if a big tsunami struck the coastline, said most residents were well aware of the risks and accepted it as the price they paid for living in a coastal area.

"You can't live in fear; you just have to live for each day and make the most of it," she said.

The council has just completed the installation of tsunami warning sirens at an additional 25 sites along the Christchurch coastline. It brings the total number of sirens in place from Spencerville to Taylors Mistake to 47.

The locations for the sirens have been selected based on the recent tsunami modelling. The sirens are only designed to be heard within a few blocks of the beach.

* The council is running a series of seminars this month for people interested in learning more about coastal hazards and how they plan to deal with them. Details on where and when the seminars are being held can be found on the council's website.