Walton-Jones said western areas would get the heaviest rain, while eastern parts of the South Island would get the worst of the wind.
In Wellington, the weather will give returning holidaymakers a rough welcome home, with an orange strong wind warning in place from 6pm Monday until 10am Tuesday.
Rain will follow and linger until about mid-afternoon on Tuesday, Walton-Jones said.
Exposed roads such as State Highway 2 over the Remutaka Hill and the Wainui Saddle on SH1 Transmission Gully could make for a challenging final stretch home.
Drivers should watch for fallen trees, power lines and wind-blown debris. Trucks, vans, campervans and motorcycles are especially vulnerable to strong gusts, Walton-Jones said.
A strong wind watch covers Wairarapa, while a heavy rain watch is in place for the Tararua Range.
Across Cook Strait, Marlborough’s strong wind watch runs from 5pm Monday until 7am Tuesday.
Affected routes include SH1 between Picton and Kaikōura, SH6 between Blenheim and Hira, SH62 from Spring Creek to the SH6 junction, and SH63 between Renwick and St Arnaud.
Further south, the Westland ranges are in for a soaking, with 200-300mm of rain forecast in places under an orange warning beginning at 6am Monday.
Fiordland’s orange heavy rain warning runs from 2am until 9pm Monday, while 140-180mm is expected in the headwaters of Otago lakes and rivers.
The heavy rain could quickly swell streams and rivers, with surface flooding and slips making roads hazardous.
Otago also faces a strong wind warning from 11am until 10pm Monday, with watches and warnings covering parts of Canterbury, the Grey District and the Buller District, Southland and Stewart Island.
Christchurch is under a wind watch, although Walton-Jones said Banks Peninsula was expected to take the strongest gusts.
Motorists travelling through affected areas are being urged to allow extra time and check conditions before setting off, as bad weather could shut roads at short notice.
It is a sharp change from Saturday’s warmth, when Christchurch, Blenheim, Napier and Gisborne all climbed above 25C.
The front should move away on Tuesday evening, but spring will keep things unsettled, with westerly winds and showers following through the rest of the week.
NZTA predicts busy northbound traffic on SH1 between Ashburton and Christchurch from 12.30pm to 6pm on Labour Day. Graphic / NZTA
NZ Transport Agency Waka Kotahi (NZTA) urged drivers to slow down, increase following distances and use headlights in wet conditions.
Motorcyclists, those towing, and drivers of high-sided vehicles like campervans should take care on exposed roads and pull over safely if conditions become challenging.
Motorists should avoid floodwaters and pull over somewhere safe if strong winds make driving unsafe.