The Canterbury Regional Council says people should think carefully about heading into the back country, as heavy rain could make rivers rise quickly.
MetService says rain is likely to fall on much of the country, particularly South Canterbury and North Otago, on Sunday.
The council says about 130mm of rain is expected to fall on the foothills, with about 50mm on the coast.
Duty controller Tony Henderson says rivers will rise quickly if it falls at a rate of about 10mm per hour. Surface flooding and road damage could occur.
Stock should be moved to higher ground and people should rethink plans to head into the back country.
MetService says persistent rain is expected to last until Monday evening in Canterbury and eastern Otago.
Heavy snow above 900 metres is also likely.