At the same time, a low from the Tasman Sea would pass onto the North Island, bringing periods of heavy rain and strong northwesterly winds to the North Island.
By Saturday night the southerly system that brought snow to the South Island would move into the lower North Island, bringing strong gales and snow to the lower North Island.
"Either one of those systems is significant on their own, but to have both, it's quite a significant weather day for all of New Zealand," Mr Kerr said.
"The few redeeming features of the systems is that they're relatively fast-moving."
The heavy rain for the North Island would be short-lived for most places, but the Eastern Bay of Plenty could expect the 80mm of rain in a 12-15 hour period from midday Saturday that could cause surface flooding and rivers and streams to rise quickly.
The snow in the South island could fall heavily in Southland, Fiordland Canterbury and Marlborough where 10cm to 15cm could accumulate down to 300 metres.
Mr Kerr said he expected the snow fall to close roads above 400 metres in the South Island and lower North Island.
"People should stay alert and try to modify their journey or change their travel plans."
Southerly winds would arrive in Wellington with "quite a bang" on Saturday night and into Sunday, Mr Kerr said.
Severe gales and gusts of 120 km/h are expected for the capital.
In Auckland, Saturday will be wet and windy, rain will become heavy around midday and some possible thunderstorms in the west.
The rain will ease Saturday night, Mr Kerr said, and the northerly winds could be gale-force.
On Sunday, the winds will change to southerlies and could also be gale-force.
Weather forecast
Whangarei: Heavy rain tomorrow morning will turn to showers by evening. Wind gusts of up to 100 km/h tomorrow will turn southerly on Sunday. Highs around 15C.
Auckland: Rain with some heavy falls tomorrow turn to showers by evening and into Sunday. Wind gusts up to 90 km/h and turn southerly on Sunday. Highs around 15C.
Hamilton: Rain with some heavy falls tomorrow. Strong winds turn southerly on Sunday and rain eases. Highs around 13C.
Tauranga: Rain with some heavy falls tomorrow and some strong westerly winds. Becoming fine with southerly winds on Sunday. Highs around 15C.
Wellington: Rain develops tomorrow and strong northerlies turn southerlies by evening with gusts up to 120 km/h and severe gales. Rain eases to showers on Sunday with gale-force southerlies and snow around hills. Highs around 10C.
Christchurch: Rain and possibly sleet developing tomorrow with southerly winds and heavy snow above 400m. Sleet on Sunday morning turns to occasional showers and strong southerlies. Highs around 9C.
Dunedin: Sleety showers developing tomorrow and southerlies. Sleet on Sunday morning turning to occasional showers and strong southerlies. Highs around 6C.
