A MetService spokesman said an active front, preceded by strengthening northwest winds, would move eastwards across the South Island tomorrow, bringing heavy rain to the west of the South Island and to the Otago and Canterbury headwaters.
"Gale to severe-gale northwesterlies preceding the front are expected to affect the south and east of the South Island and the lower North Island.
"Heavy rain and strong wind warnings and watches are in force for these areas."
Orange heavy rain warnings are in place for the headwaters of the Otago lakes and rivers, from 4am to 2pm tomorrow, when up to 140mm is expected to fall near the main divide; and up to 110mm is expected to fall within 25km east of the main divide.
Thunderstorms are possible and peak rates of up to 30mm/h of rain may fall.
In Fiordland (about and north of George Sound), up to 110mm is expected to fall between 11pm tonight and 10am tomorrow.
Peak rates of up to 30mm/h may fall during possible thunderstorms.
Heavy rain may cause streams and rivers to rise rapidly, MetService said.
Surface flooding and slips are also possible and driving conditions may be hazardous.
A heavy rain watch is in force in Fiordland (south of George Sound), from 11pm tonight until 10am tomorrow.
Further rain or showers are expected from Tuesday afternoon through to early Wednesday morning.
Strong wind watch
A strong wind watch is also in place for all of Otago (except Clutha) from 11pm tonight until noon tomorrow; and in Fiordland, Southland, Stewart Island and Clutha from 9pm tonight until 9am tomorrow.
"Northwest winds may approach severe gale in exposed places," a MetService spokesman said.
He advised residents to keep up to date with the latest forecasts in case parts of it were upgraded to Orange Warnings, or further areas were added.
Heavy rain for West Coast
A heavy rain warning is also in place for the West Coast from 4am until 6pm tomorrow.
For the ranges of Westland about and south of Harihari, 130mm to 170mm of rain is forecast, with possible thunderstorms.
Further rain or showers expected from tomorrow night until early on Wednesday morning.
