Cyclone Donna may now be just a Category 1 storm but she will still bring heavy rain to parts of New Zealand before the end of the week.

Donna hit Vanuatu as a Category 5 cyclone, making her the worst tropical storm ever to hit the South Pacific in May.

She has now weakened to a Category 1 as wind speeds ease but is expected to reintensify.

MetService forecaster Georgina Griffiths told NZ Herald Focus a Tasman low was moving toward New Zealand and many Kiwis could expect a "rough and tumble end to the working week".

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"We have a mature low over the Tasman Sea and that's going to be the initial rainmaker. then we have the remains of Donna, who is coming out of the tropics and she's rapidly weakening."

The direction of the low is hard to predict but MetService is now saying it is likely to affect Nelson and the West Coast of the South Island.

Meanwhile the remnants of Donna will move south towards New Zealand, bringing further heavy rain. The storm is expected to pass just off the East Cape in the North Island around midday Friday.

MetService has issued heavy rain warnings for the Nelson and Bay of Plenty regions.

Niwa said the upper and western North Island could expect the heaviest rain, with some areas in for more than 100m between now and Friday morning. Ponding on roads and localised flooding is possible, Niwa said.

As Donna weakens, another late-season cyclone named Ella is slowly tracking toward Fiji where it is expected to bring heavy rain. Cyclone Ella is not expected to head for New Zealand at this stage.