The rain has closed some roads, prompted slip warnings and wastewater overflows have contaminated floodwater. All floodwater should be considered contaminated.
Christchurch City Council's head of Three Waters Helen Beaumont said some garages and baches have flooded in Le Bons Bay.
"The Heathcote and the Avon rivers have flooded across roads in places, and we've got a bit of surface ponding from these heavier falls from earlier in the evening ... There are multiple road closures across the city. But it's spread out across the flatland across the city rather than in any one place," Beaumont said.
However, some areas have been hit worse than others.
"Right along the Avon River corridor is reasonably hard hit, and we've had a reasonable number of road closures there ... Out along the east, we've had some very heavy rain along the coast, so some areas of South New Brighton, down Pages Road has been hard hit."
The rain had moved south of the city by late Wednesday, but caution should be exercised, she said.
Christchurch City Council said rain had eased and council contractors would be out at first light to assess damage.
The council wanted people to minimise toilet flushing and save showers for the morning.
In Selwyn, a water conservation notice has been issued for Springfield residents.
The district council said that was due to cloudy water from heavy rain and low reservoir levels.