A severe thunderstorm warning has been issued for the Canterbury Plains and North Otago, with heavy rain and hail likely to batter the regions.
Other parts of the country are set to be whipped by severe gales.
MetService has warned people in the Canterbury Plains and North Otago to prepare for flash flooding around low-lying areas such as streams, rivers or narrow valleys, which may lead to slips. More than 25mm of rain is expected in some areas.
A front is moving eastwards across the South Island overnight.
This means a burst of heavy rain with thunderstorms possible in the Westland ranges south of Otira, and in Dunedin and North Otago.
Driving conditions will also be hazardous, with surface flooding and poor visibility in heavy rain.
Large hail can cause significant damage to crops, orchards, vines, glasshouses and vehicles.
Northwest winds may reach severe gale from inland Canterbury to Marlborough, Wellington and Wairarapa.
Meanwhile, a storm likely to bring damaging winds and huge waves is centring on New Zealand's West Coast, sparking warnings to those camping, tramping or on the water.
Weatherwatch.co.nz is forecasting "damaging" gales over 150km/h, waves up to 13 metres, and a metre of snow on the Southern Alps.
There could also be more than 200mm of rain for parts of the West Coast.
Police said they hadn't issued a specific warning for the storms, but they always urged motorists to drive to the conditions.
"In wet and windy weather that means reducing speed and increasing following distances," a spokesperson said.
Philip Duncan of weatherwatch.co.nz said the storm would be significant.
"A stormier Southern Ocean weather pattern is temporarily putting the La Nina pattern to one side with two significant lows - one today and the other around Tuesday, Wednesday.
"The Sunday low, which is still yet to push in some thunderstorms, rain and wind for parts of New Zealand, will actually be tracking away from the country today. So we don't expect anything too serious today, although keep up to date with any possible MetService severe warnings where you are."
But the next storm has Duncan concerned, especially for those adventuring outdoors.
"The Tuesday/Wednesday event looks to be most intense with the centre of this storm potentially crossing Southland and Otago."
As it surges up the country with its strong northwest gales, it would then be followed by a cool southerly change with heavy rain set to lash the West Coast.
Auckland is tipped to cool down but won't experience gales as harsh as those in the South Island.
MetService meteorologist Peter Little said the southwest change kicking in on Wednesday into Thursday would drop temperatures by up to 10C in the South Island.
Dunedin will drop from 25C today to 15C.
Little said the temperature change wouldn't be as dramatic in the North Island, but people would definitely feel the impact of the southwester.
Auckland will drop from 27C today to 21C on Wednesday, and 20C on Thursday.
Until then, the hot weather will continue. Whangārei and Gisborne can expect 30C, Auckland and Tauranga 27C and Hamilton 26C.
In the South Island today, Kaikoura is the hot spot with 28C. Christchurch and Ashburton are set for 27C.
The front is expending its energy in the South Island, and central New Zealand - Wellington, Wairarapa - are bearing the brunt of the strong winds. Severe weather warnings have been issued.
-Additional reporting RNZ