The cause is a "pipeline" of moisture coming down from the tropics and feeding over central New Zealand.
Cyclone Dovi is expected to arrive from New Caledonia onto central New Zealand sometime on Sunday. It's likely to cause some big swells and hazardous cross and rip currents against the west coast of the North Island.
These could be mainly north of Taranaki, Corrigan said, but forecasters were still expecting high waves to come down into the South Taranaki Bight and affect Whanganui.
Winds could also be gale to severe gale force in parts of the central North Island.
Horizons' acting group manager of river management Craig Grant said the uncertainty about how much rain there would be meant staff needed to be ready.
Mr Grant said catchments across the region recovered reasonably well after the rain last weekend.
"However, we will see river levels rise again in response to any significant rainfall and are preparing for possible Makino and Moutoa gate operations.
"Spillway leaseholders will be contacted as necessary and we're in touch with our city and district councils to make sure they're prepared for any local impacts as a result of the rain."
Grant said there was also concern there would be strong winds in Whanganui and Horowhenua from late Saturday.
Despite the rain, temperatures will continue to be warm with a high of 27C for Saturday and 25C for Sunday, Corrigan said.
Overnight temperatures will be 19-20C, making sleeping difficult.
Humidity will continue being high, with a dew point of 20C.
That made the air feel even hotter, Corrigan said, because the natural cooling process for human bodies was to sweat.
"In high humidity sweating isn't as effective at cooling you down."
Winds are to turn from northerly to southeasterly and to stay there, strengthening on Sunday.
Up-to-date information on river heights and rainfall is available via Horizons 24 hour toll-free Waterline 0508 4 FLOOD or via our website www.horizons.govt.nz
Updates will also be posted to the Manawatū-Whanganui Civil Defence page www.facebook.com/civildefencemanawatuwhanganui/
Early next week the weather was expected to return to "normal summer", Corrigan said. Daily highs would be in the 22-25C range, and the overnight low on Sunday night drops to 13C - better for sleeping.
