An infestation of rodents and feral cats in quake-ravaged areas of Christchurch is inevitable but authorities can do little to prevent it.

Pest experts say rats, mice and cats - both wild and those that ran away or were abandoned after last month's quake - are likely to become more visible in vacant houses and the CBD.

Royce Delaney, from Pest Control Services, said the central city and some suburbs near the Avon River had always had rodent issues. But the quake forcing people from their homes and preventing control measures being applied exacerbated the problem.

"The main concern for the CBD is the river. Rats live and breed around the river banks. There has always been an infestation of rodents in Christchurch and there have been control programmes put in places that are serviced monthly," he said.

"Obviously that's not in place any more. It's inevitable that if nothing is done the mice and rats will just breed."

He said rats needed only 21 days to breed and produced up to eight babies each birth. Mice needed just 19 days and had up to 12 babies. "If left long enough it will be an issue. It is inevitable they will become a problem over the next three or four weeks."

Mr Delaney said rodents were attracted to empty buildings.

"It's not so much the rubbish that's the problem. The situation has changed, houses usually full of people are now vacant. That attracts rats and mice into new areas. They go under the houses and no one is there to recognise it or put controls in place.

"They will eat anything from grains and fruit to meat. If they are hungry enough they will eat each other."

He said areas of the CBD heavy with rescue workers would remain rodent-free for longer than those abandoned.

In the suburbs, cats would also be a problem if they were producing wild kittens.

A spokeswoman for the environmental health team working from the Civil Defence headquarters within the CBD cordon said rodents were at nuisance level

"There is a waste management issue within the cordon. There is a plan in place but because the area is inaccessible to environmental health staff, they are not able to go in and carry out that plan at present," she said.

"It is a public health nuisance at the moment, rather than a public health hazard at this point."

She said the safety of environmental health workers had to come before waste management and rodent issues.

"Human safety comes before anything else. But where a response can be carried out safely, it will be."

Things were slightly different in the suburbs. A homeowner with a rodent problem in a red-stickered house could get pest controllers in if their insurer or building assessor deemed the property safe to have work carried out.