The blaze, which began six days ago near the city of Nelson, is now threatening the town of Wakefield.
A state of emergency has been declared and about 3,000 people have fled their homes in the district of Tasman.
Strong winds were expected, and officials warned that Sunday could be a "critical danger point" for the fire.
Prime Minister Jacinda Ardern said she hoped "the weather plays ball".
The blaze is thought to be the worst forest fire in New Zealand since 1955.
Nelson MP Nick Smith said the entire region was a "tinderbox" and its 70,000 residents were "on edge."
Twenty-three helicopters and two planes have been deployed to tackle the blaze. Rain forecast for the area on Tuesday is expected to miss the fire zone.
Fires of this size are unusual for New Zealand, with local media calling it the worst bushfire in 50 years.