Cyclone Percy is continuing to intensify as it moves towards Palmerston atoll in the Cook Islands.
The cyclone, with winds of 195 kilometres an hour and gusts of 260 kilometres, should pass to the west of Palmerston late tonight as it continues on its southerly path.
Jeff Sanders, the lead forecaster at the New Zealand Met Service, says there will be frequent heavy rain and squally thunderstorms as well as the high winds.
And, he says another danger will come from damaging sea swells.
"Obviously, it's a slow moving system. We're getting a lot of swell built up in that process, and flooding, especially the low lying coastal areas, expected. These increasing winds, or the cyclone moving southwards, will also affect the rest of the southern Cooks too."
Cyclone Percy is expected to pass to the west of Rarotonga and Aitutaki.