Temperatures are expected to reach more than 30 degrees in parts of the country this weekend.

The South Island appears hottest with temperatures approaching 30 degrees from Southland to Otago to Canterbury to Marlborough, says WeatherWatch.co.nz

Inland areas will be hottest with highs currently forecast by WeatherWatch.co.nz/IBM of 28 to 31 degrees. Hawke's Bay up to Gisborne may be similar too.

Southland, which has another cold and sometimes wintry week on the way, has a dramatic change this weekend with sunny weather and highs in the late 20s.

As WeatherWatch.co.nz has been saying for weeks, October is a very unsettled month with plenty of reminders winter has just finished, but not November has more high pressure and more hints that summer is just around the corner.

"This weekend is the first time in several months that New Zealand has temperatures forecast to reach 30 degrees.

"This weekend looks hotter than normal in every part of New Zealand - and drier than average too.

"But not every region will be "hot". Our largest city Auckland has some great weather this weekend - but highs range from 18 to 24 degrees across the region. New Plymouth is similar.

"Wellington also has a fairly average weekend although does tip warmer than average on Sunday but Saturday may only make it to 17 degrees."

Dunedin has highs in the early 20s.

WHAT IS CAUSING THIS HEAT?

The hot weather is being caused by a large high in the Tasman Sea. The anticyclonic flow means sub-tropical air is being pushed down over hot inland parts of Australia then directed our way.

New Zealand's hottest days are when we have a direct air flow from Australia's interior like we do this weekend.

"If this set up occurs in mid-summer we could be hitting over 40 degrees, which has happened a few times in NZ's past and has broken heat records."

Why is it hot this weekend? You can thank both Australia and the sub-tropics.