Civil Defence is encouraging Christchurch residents to continue using chemical toilets as wet weather puts extra pressure on the earthquake-damaged sewage system.
National controller John Hamilton says residents in all suburbs - damaged or not - should conserve water and flush toilets sparingly.
Mr Hamilton says those in the worst-hit eastern suburbs due to the quake on 22 February should also continue to use chemical toilets and port-a-loos if they are available.
The system is badly damaged and not able to cope with large volumes of waste water, he says.
At present, about 70% of it is going to waste water plant, while the rest is overflowing into rivers and estuaries.
The Christchurch City Council is asking people to go easy on essential services as they are at risk of collapse.
Residents are urged to continue using portable toilets as the council works to avoid odour problems from untreated sewage.
Silt from the quake has collected in the main sewers and waste water is flushing it into the treatment plant at Bromley.
The council's water and waste manager, Mark Christison, says the silt is clogging the plant and forcing engineers to pump untreated sewage into oxidation ponds.
Mr Christison says silt is being removed from the sewers and the pumping station, but it could be four to six months before all is cleared from the network.