The sunshine that has smiled down on Aucklanders is set to stick around for the start of the week - but a "rainmaker" is on its way.

A ridge of high pressure that covers much of the country is forecast to last through to Tuesday.

Auckland is forecast to reach a high of 20C on Monday, Tuesday and Wednesday amid partly cloudy skies.

MetService meteorologist Melissa Oosterwijk said the high pressure system would move slightly eastwards as the week wore on.

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In this case, for Auckland the forecast is for really fine days with fog at nighttime when it gets cold, she said.

The settled weather follows on from a pearler of a day on Sunday in which most areas around Auckland experienced 18C or 19C.

The top temperature Sunday was in the Napier and Hastings area, with tops of 21C.

Not far behind was Blenheim with a 20.5C high.

However, a trough should approach the country from the Tasman Sea on Wednesday.

According to WeatherWatch.co.nz the warmer than average week ahead could end by the time the weekend rolls around when potentially a sub-tropical rainmaker comes through followed by a cooler southerly.

"Some long range data does suggest a large Tasman Sea low may affect New Zealand perhaps next weekend (not this weekend) as certainly energy is building to out west over the coming week for a low and rain-maker to grow," WeatherWatch says.

WeatherWatch.co.nz also warned of fog throughout inland areas over the next several days, especially over the North Island - this could disrupt more flights.

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Fog can sometimes be tricky to forecast so be prepared for possible "sudden fog", the forecaster said.

In these conditions, as in Waikato on Saturday, fog and could linger as late as early afternoon.