The MetService says Cyclone Evan might bring heavy rain and severe gales to Northland on Saturday.
After cutting a devastating path across Samoa, Evan is now pounding areas of Fiji with high winds and rain.
The MetService said Evan was expected to move away from the main Fijian island of Viti Levu after midnight tonight.
The cyclone was expected to undergo its mid-latitude transition late in the week or during the weekend as it approaches the north of New Zealand.
"There is still a lot of uncertainty in the forecast strength and track of the resultant low over the weekend," the MetService said.
Before then, the warm weather of the past few days was expected to continue for the North Island much of this week, with cooler days expected in the far south.
Rain or showers were forecast for the whole country tomorrow, with clearer weather for most on Wednesday and Thursday.
"The air currently flowing over much of New Zealand has come from the tropics," chief forecaster Peter Kreft said.
"This air holds a lot of moisture, and through to the latter part of Tuesday, most places should see some rain."
A cold front in the South Tasman Sea was expected to move on to the southern South Island this evening, while a warm front in the north Tasman Sea should move onto the northern South Island tonight.
A moist northerly flow ahead of those fronts is expected to bring heavy rain to the western South Island today, MetService said.
Falls of up to 180mm were possible in Fiordland today, with up to 100mm about Buller.
A high should develop over most of the country during Wednesday and move slowly east on Thursday.
Warm weather during the past few days included a top temperature of 29degC at Masterton yesterday and 30degC in Christchurch and Hastings today.