Best said La Niña's continuing influence on New Zealand weather means temperatures are above normal.
"The tropical flow means that the North Island experiences a lot of warm, northeasterly breezes."
Whanganui is forecast to experience a northwesterly change on Thursday, bringing cloud and evening rain with a high of 25C.
"Friday will have periods of rain throughout the day and the northwesterly is forecast to turn southeast in the afternoon," Best said.
"It will still be warm in Whanganui - the high on Friday is still 23C."
The forecast for Saturday is mainly fine with the chance of an afternoon shower and light winds.
Showers are forecast for Sunday and the high on both days will be 24C.