As well as Whanganui, that included Nelson, Marlborough, Taranaki and Manawatū, McInnes said.
"That [clear weather] won't really be around to stay - it sort of will right through the weekend.".
The southwesterlies would make way to northerlies, he said.
"As we get into the working week we might see cloudier, possibly even wet days.
"That will be a noticeable change in the weather regime compared to what we've seen over the last week."
McInnes said it had been colder than average, but that was to be expected because at night there were no clouds to stop cooling weather.
The persistence of the good weather at this time of year was not very common, McInnes said.
"We're more used to getting fronts and the like during this time of the year."