Hastings looks set to bear the brunt of this weather with the heaviest amount of rain expected to fall between 6am and 11am on Wednesday, peaking at 6.9mm at 10am.
Napier's rain will also peak around that time at 4.6mm, with the heaviest rainfall between 4am and 10am.
"Wednesday will be a wet day."
He cautioned, however, that the current forecast was only "indicative" and things might change.
Thursday would be mostly cloudy, with showers developing in the afternoon and clearing in the evening as nor'westerlies tend southwest.
While no severe weather warning had been issued for the region, McInnes said it would be worth watching the Ruahine Ranges, where a weather watch was in place.
"They're a bit of a moisture trap and are likely to get more significant amounts of rain."
There may also be a few thunderstorms about the ranges from the afternoon and further north of the bay, where the weather was more active.
"The main thing is to keep up to date with watches and warnings.
"This is a situation which can evolve."
He said the areas worst hit by the front would be Taranaki and Bay of Plenty, which had been issued with a heavy rain warning.
Motorists were advised to take care if travelling along State Highway 5 between Napier and Taupo.