Whanganui district looks likely to get its share of unsettled weather predicted across the country for much of the week.

MetService forecasters say the unpredictable patterns are arriving on a series of active low-pressure troughs and associated fronts passing over the country.

Tomorrow, a front moving east across the North Island is expected to bring a period of rain with heavy falls in western and central parts of the country, plus northwesterly gales in central New Zealand.

While rainfall could reach warning levels further south, forecasters are saying Whanganui could cop some of the northwesterly gales from Taranaki and Taihape southwards.

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On Wednesday, another active low-pressure system slowly moves eastwards across the South Island and is expected to bring heavy rain from eastern Taranaki and the central plateau. That risk will linger through Thursday, but at the same time strong, cold southerlies should be arriving here.

The cold south-to-southwest flow over the country eases on Friday as a narrow ridge of high pressure moves in from the Tasman Sea.

Coupled with the topsy-turvy weather is the brightest supermoon in 68 years, which will cause king tides throughout the country when it lights up the sky tomorrow, creeping over the eastern horizon at 8.45pm.

Supermoons occur when a full moon lines up with the sun, coinciding with the point in the moon's elliptical orbit when it is closest to Earth. The effect is a greater gravitational pull on the ocean, which results in king tides - the highest tides of a given year. They usually occur between four and six times a year.

But MetService is not forecasting exceptional tidal events because its spokesperson said the difference between a regular full moon tide and a king tide was negligible.