Cyclone Gita, the tropical cyclone that hammered Tonga, is expected to head to New Zealand this week.
MetService meteorologist Lisa Murray says there'll likely be heavy rains and surface flooding north of Haast and Oamaru.
"There will be some areas that can expect to see gusts of 150km/h - or even possibly more."
She says we can expect coastal inundations and storm surges.
"We'll probably start seeing some rain late on Monday, but the heavier stuff will start late on Tuesday, into that Wednesday period."
The Tasman District is powering up its emergency operations centre on Monday with a warning for Mapua and Ruby Bay residents to be prepared to evacuate if necessary.
Civil Defence says Gita has the potential to pack a punch, so people need to be prepared.
MetService says the tropical cyclone is unpredictable and its path could easily change.