Police are urging drivers to take care, with severe weather warnings in place for several parts of the South Island.
Heavy rain is anticipated for the West Coast, with as much as 240mm of rain expected between now and Friday evening.
Thunderstorms on Friday morning might bring up to 35mm of rain per hour.
Southland, Fiordland, Otago and Canterbury, meanwhile, can expect blustering northwest gales on Friday, up to 160km/h in exposed inland places.
Marlborough, where a large forest fire has been burning since yesterday near Blenheim, is also expecting gusts of 130km/h - and possibly 160km/h in some areas - tomorrow.
The run-on effects of the rain and gales have the potential to be significant: surface flooding from rising river and stream levels is possible, as are falling trees, powerlines and damaged irrigation systems.
A police spokesperson said road users in Southland needed to be aware of the potential for heavy rain and gusts, which would mostly affect those on motorcycles and those in high-sided vehicles.
There was an expected increase in people on roads around the region with the Burt Munro Challenge beginning today, the spokesperson said.
The challenge, with various events around Invercargill, was created by the Southland Motorcycle Club to honour Munro, who set land speed records in the 1960s.
Wild winds are also expected in Wellington and southern Wairarapa in the North Island, with severe northwest gales rising to 130km/h in exposed places on Friday.