MetService said the heaviest rain is likely in the West Coast and Tasman District ranges, with thunderstorms possible there too.
Heavy rain warnings have been issued on Sunday for Tasman northwest of Motueka from 11am, the Paparoa Range and the ranges of the Buller District south of Seddonville and northwest of Reefton from 2pm, and the ranges of Westland from 9am.
Heavy rain watches are in place on Sunday for Richmond and Bryant Ranges, the Canterbury headwaters, the Tararua Ranges and Mount Taranaki/Taranaki Maunga.