A severe weather warning has been issued for Westland, and a watch has also been issued by MetService for Canterbury headwaters for Tuesday and Wednesday.
The heaviest falls are forecast for the ranges of Westland south of Otira, where up to 150mm of rain may fall tomorrow. MetService also says rivers and streams may rise rapidly, with possible slips and surface flooding.
An active frontal system over the Tasman Sea is expected to move onto the South Island overnight, then slowly northwards across Canterbury on Tuesday.
A period of heavy northerly rain is forecast for the Southern Alps ahead of this front, where the severe weather warning is now in force, MetService says.
The heavy rain is likely to hit the headwaters of the Canterbury lakes and rivers south of Arthurs Pass on Tuesday.
