A magnitude 8.1 earthquake struck near the Kermadec Islands on Wednesday morning, triggering a widespread tsunami alert throughout the country.

This was the scenario facing staff at Whanganui Hospital during Operation Tangaroa, a nationwide exercise which tested the country's readiness for, and reaction to, the impact of a tsumani.

Civil Defence, councils and district health boards throughout the country took part in the exercise.

"We are anticipating potentially hundreds of casualties, but we don't yet know how the hospital itself will be affected," said Simon Ward, the Whanganui District Health Board's emergency manager said.

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He said the call came from the Ministry of Health about 9.20am on Wednesday, and the hospital's emergency operations centre [EOC] immediately swung into action under a code red - the highest alert rating.

While the hospital operations continued as normal, around 20 staff manned the EOC, located on the fourth floor of the hospital. These included staff from communications, administration, IT and some medical staff.

"These are the people who would normally be involved in an incident like this. It means that the medical staff are free to get on with the work that they need to do," Mr Ward said.

He said new challenges were regularly thrown at the team throughout the day that they had to respond to.

Mr Ward said the exercise would continue over September. In two weeks' time will be a "day after the tsunami" scenario; while a "two weeks after the tsunami" scenario will be held in late September.