MetService says the heavy rain is moving away from Westland on Wednesday evening and heading north to Buller and Nelson.
Nearly 600 millimetres of rain has fallen on the West Coast, closing State Highway Six at Harihari after a bridge was washed away at one end.
Arthur's Pass between Bealey and Jacksons is also closed because of flooding.
MetService forecaster Andy Downs says thunderstorms heading towards Buller and north west of Nelson could lead to flash flooding and damaging winds.
Mr Downs says the front is likely to clear from the region by midnight on Wednesday.
He says it should weaken as it moves north, however, severe wind warnings are in place for the lower North Island, inland Canterbury and Marlborough.