Heavy rain's forecast for parts of the South Island tonight, before they move across the country

MetService says an active front over the Tasman Sea's likely to move east through to Friday, with strong falls in many areas and heavy snow in the south of the South Island.

The Westland District has an orange heavy rain warning from 11 tonight.

Meteorologist John Law says more watches and warnings come into effect throughout the day tomorrow.

He says the rainfall will arrive on western parts of the South Island by the end of today, and heading into Wednesday it'll head northwards.