Ambulances are out on the streets as normal in Christchurch amid the devastation two days after Tuesday's earthquake, but with their stations destroyed many St John staff are operating out of a carpark in the city centre.

Ambulance workloads were not high but every job took a lot longer than normal because it was difficult to move around the city, St John chief executive Jaimes Wood said.

With Christchurch's main ambulance station wrecked in the September 4 earthquake, and the new station where the service relocated to destroyed in the latest quake, St John was now operating out of a central city carpark in front of the previous station.

"Of our six operational ambulance stations in Christchurch, three are quite damaged beyond use, so it's not just the main station, it's elsewhere as well -- and the problems there are again no food and water or hygiene facilities for our people," Mr Wood said.

Volunteers were looking after staff, and the community had been "absolutely wonderful".

"We've got to keep our people fed, watered, and keep morale up and keep the show on the road to serve our communities."

There were ambulance staff whose houses had been destroyed and had only the clothes they stood up in, and several volunteers were uncontactable.

Over 161 St John frontline medical staff have been sent to Christchurch. In addition to 40 ambulances in the city, St John had deployed 20 aircraft, 12 mass casualty vehicles and 10 support vehicles, and flown more than 200 patients from Christchurch to other hospitals.

"Obviously this is early days, and there's going to be a long haul -- we're talking weeks for sure if not months and keeping people motivated, keeping people fresh enough and recycling resources in and out of what is a really tough set of circumstances, those are the issues we're going to face," Mr Wood said.

Staff were responding to high numbers of heart problems, chest pains, breathing problems and traumatic injuries, as well as frightened people who wanted reassurance.

St John staff were stationed at the welfare centres at Burnside High, Pioneer Stadium, Cowles Stadium and Rangiora.

A lot of issues were going to start arising at welfare centres, where there were people without their medication, and no water and no sewerage.

Diarrhoea and vomiting were going to be a challenge, so it was vital to use routines such as keeping toilets separated, washing hands and general personal hygiene.

"All of that relatively basic but very effective risk mitigation stuff."

St John had received offers from ambulance services in Australia, of staff and ambulances, and from the London ambulance service, but had not had to take them up yet.

St John staff were generally coping well, although there was a wide range of responses.

"We've got a really good team, I think our reponse has been exceptional," he said.