MetService said the heaviest rain was expected around the ranges of the Westland District, where an orange heavy rain warning had been issued.
Locals could expect 120-150mm of rain, with peak rates expected in the afternoon.
"Clear your drains and gutters to prepare for heavy rain. Avoid low-lying areas and drive cautiously," the forecaster said.
Heavy rain watches were in place for wide swathes of the South Island, while road snowfall warnings had also been issued in the bottom part of the country.
Heavy rain watches
The ranges of the Grey and Buller Districts, from 6pm Sunday to 6am Monday.
The headwaters of the Canterbury lakes and rivers from Arthurs Pass southwards, from 12pm-9pm Sunday.
The headwaters of the Otago lakes and rivers, from 3am-4pm Sunday.
Fiordland north of Secretary Island, from 9pm Saturday to 3pm Sunday.
Inland Southland north of Winton, from midnight Saturday to 3pm Sunday.
Orange heavy rain warnings
The ranges of the Westland District, from 6am-midnight Sunday.
Road snowfall warnings
Arthurs Pass (SH73)
Valid: 4 hours from 11pm Sun 25 Aug to 3am Mon 26 Aug
Forecast: Snow is possible overnight Sunday. Less than 1 cm is expected to accumulate about the road above 900 metres.
Porters Pass (SH73)
Valid: 4 hours from 11pm Sun 25 Aug to 3am Mon 26 Aug
Forecast: Snow is possible overnight Sunday. Less than 1 cm is expected to accumulate about the road above 900 metres.
Lindis Pass (SH8)
Valid: 3 hours from 8pm Sun 25 Aug to 11pm Sun 25 Aug
Forecast: Snow is possible Sunday evening, where 1 cm of snow may settle on the road above 800 metres.
Crown Range Road
Snow is no longer expected.
Milford Road (SH94)
Snow is no longer expected.
