The national forecaster said a strong, moist northerly flow would bring intense rainfall and damaging winds before conditions turned showery with westerlies later on Monday.
Orange heavy rain warnings are in force for several regions.
They include the headwaters of Otago lakes and rivers, with 120mm to 160mm of rain expected during Monday.
In Fiordland, about and north of Doubtful Sound, MetService is warning of 100mm to 150mm of rain between 6am and 4pm tomorrow.
The headwaters of Canterbury lakes and rivers south of Arthur's Pass could see up to 180mm near the main divide from 3pm on Monday until 3am on Tuesday.
In the Westland ranges, between 160mm and 200mm of rain is forecast from 9am tomorrow until 3am on Tuesday. Peak rainfall rates of 20mm to 30mm/h are expected.
MetService warns streams and rivers may rise rapidly, with surface flooding, slips and difficult driving conditions possible. People in affected areas were advised to clear drains and gutters, avoid low-lying areas and take care on the roads.
Strong winds
Strong wind warnings are also in place, with Fiordland facing severe gale-force north to northwest winds gusting up to 120km/h from 4am until 2pm tomorrow.
Similar conditions were expected in the Canterbury High Country from 9am tomorrow, with damaging winds likely until 3am on Tuesday.
Damage to trees, powerlines and unsecured structures is possible, with the strong winds making driving hazardous, particularly for high-sided vehicles and motorcycles.
A yellow strong wind watch also covers Queenstown Lakes, Central Otago and inland parts of Southland, Clutha and Dunedin, throughout Monday, where winds may approach severe gale strength in exposed areas.
