Weatherwatch.co.nz head analyst Philip Duncan said the storm would rapidly deepen and intensify before it made landfall.
Campers and trampers were warned to be aware of the deteriorating conditions.
MetService meteorologist April Clark said there was a heavy rain warning out for Rotorua and the Bay of Plenty.
Rain is expected to become heavy for a time from tomorrowafternoon to around dawn on Friday.
It is expected about 70mm to 100mm rain will accumulate between 3pm tomorrow and 6am on Friday.
Clark said there was also a strong wind warning out for Rotorua and the Bay of Plenty. It was expected the area could get gusts up to 120km/h.
"It really will feel like quite a stormy night. Hopefully everyone will be indoors and feeling nice and snuggled in."
Lake and river levels in Bay of Plenty were being closely watched, the Bay of Plenty Regional Council said in a statement tonight.
"Rivers have not reached their first warning levels and are not expected to at this stage, but farmers are advised to be mindful of stock beside smaller streams, which can rise more rapidly. Campers and others near the coast should also be extra-vigilant of possible inundation and waves."
Regional council duty flood manager Peter Blackwood said coastal areas and streams around Rotorua, Tauranga and the western Bay of Plenty were expected to be the worst affected.
"This may result in minor river flooding, and some coastal flooding particularly given recent high tides," he said. "However, based on the expected wind speed we wouldn't expect any homes to be impacted by waves and coastal flooding."
Heavy rain can cause rivers and streams to rise rapidly, as well as surface flooding and slips, so it was important people were prepared, said emergency management Bay of Plenty group duty controller Jono Meldrum.
Useful preparation tips include tying down outdoor furniture and loose objects and ensuring gutters are clear of leaves.
"A little bit of planning goes a long way," he said.
Bay of Plenty Regional Council and Bay of Plenty Civil Defence are monitoring the situation.
Rotorua Thermal Holiday Park's Andrea Scott said the Christmas and New Year period had been busy for the park and that it had been "absolutely full".
With the weather starting to deteriorate a couple of people had packed up early and people had inquired about whether changing to a cabin was possible, but all the cabins were booked.
However, people were still happy to brave the weather and pitch their tents, she said.
That includes Rhianna Swift and Jack Carling, from England, who are in New Zealand as part of a world trip.
Swift had been to New Zealand before, loved it, and wanted to come back.
She said she and Carling had a waterproof tent and could go inside one of the buildings if they needed to. They were used to the rain back in England, she said.
"We committed to tenting it."
For updates go to www.bopcivildefence.govt.nz or www.facebook.com/BOPCivilDefence.
