The trend was moving from the south and would begin easing as it moved northward, she said.
"It is very cold and it is looking like that will continue."
"As skies clear overnight it will be cold inland in the far south, cold temperatures Monday night will move north and then affect the North Island by Tuesday.
"Once showers clear then it will spell colder weather."
Down in the South Island the Lewis Pass and Porters Pass were expecting snow Monday night as snow was forecast to fall down to 500m.
There was also a chance of snow showers on the North Island's Desert Road as well as Rimutaka Road, Clark said.
Hot water bottles and extra blankets were needed last night as much of the country was set to feel the temperature drop, meteorologist Brian Mercer said.
"In the North Island we would expect places in inland Waikato to get down to about 1C and some of those higher areas like Rotorua and Taupo will get down to about 1C as well.
"In the South Island there is an improving trend overnight tomorrow and into Tuesday where they are getting some fine weather. Wanaka we are expecting to get down to -1C tonight, Alexandra and Blenheim to about 0C, and even Nelson could get down to about 2C."
The winter snap is expected to ease over the next few days; there will be a few showers but mostly fine weather on the horizon.
Tomorrow's forecast
- Auckland: Fine after morning showers. S. High 12C / Low 6C.
- Tauranga: Fine. S. High 13C / Low 4C.
- Hamilton: Fine with morning frosts. S. High 11C / Low 2C.
- New Plymouth: Morning showers heavy then fine. S. High 11C / Low 6C.
- Napier: Cloudy with showers. S. High 11C / Low 5C.
- Wellington: Showers to ease in afternoon. S. High 9C / Low 7C.
- Christchurch: Cloudy then showers, possible snow. S. High 8C / Low 4C.
- Dunedin: Cloudy with showers. S. High 8C / Low 5C.