Residents had to evacuate about 10 homes after torrential rain caused flooding in the Christchurch area on Sunday night.
Christchurch City Council's civil defence manager Murray Sinclair said Dallington was particularly bad, as was St Albans, where Dudley Creek had burst its banks.
He said contractors had been putting sandbags on roads in Lyttelton to divert water away from properties and people had evacuated around 10 properties in Lyttelton, Sumner and Akaroa after they flooded.
Earthquake-damaged roads in the eastern suburbs were vulnerable to surface flooding because the drains were not taking the water away as effectively as they used to, he said.
Dozens of homes were threatened by flash flooding in Lyttelton, after a retaining wall burst.
Residents said the wall at the top of Canterbury Street was washed out after a build up of shingle and rocks blocked storm water drains, causing a flash flood on Sunday night.
Rowena Laing described seeing a "river of water" washing down a nearby street, Fairfax reports.
Firefighters managed to divert the water away from homes.
The New Zealand Transport Agency is urging drivers to take care on State Highway 83 from Pukeuri to Duntroon and SH79 from Geraldine to Fairlie due to flooding.
And in the North Island, SH29 about 17km west of Auckland near Ruahihi Rd has been reduced to one lane due to a slip on Sunday. Motorists are advised to avoid using this road.
More heavy rain is expected to hit Canterbury and eastern Otago on Monday and Tuesday as a front moves slowly across the area.
More than 100mm may fall in some places on top of what is already "a very sodden situation", the Metservice website said.
"Members of the public are advised that streams and rivers may rise rapidly and there may be areas of flooding and slips."
Rain or showers are expected across most of the country on Monday.
News home