A big blocking high pressure system is currently covering most parts of New Zealand helping create gloomy conditions for some in the west and a few early fog and frost patches.

This high is now slowly moving eastwards, however Weather Watch says another big low is forming in the Tasman Sea which is being blocked by the high pressure system.

The front ahead of the low is expected to approach the country on Tuesday night and will tap into sub-tropical rain bands.

Bay of Plenty will experience most of the rain, with slow moving rain expected to hit the area around Wednesday and Thursday, and fine weather will continue for most parts of the country until Tuesday afternoon thanks to this high pressure.

Daytime temperatures will be significantly warmer than usual in most areas on Monday, but some may get a slight cool down again on Tuesday like Canterbury and Otago.

Over in Westland northwesterly winds bring more mostly overcast skies or even light rain or drizzle. In Fiordland heavy rainfall may occur for a time.

On Tuesday daytime, most parts of the country will still have good weather but with increasing high clouds. From the evening northwest to northerly winds will build and showery rain may occur in the western side and interior of the South Island - pushing into the North Island on Wednesday.

The main precipitation period in the northern and western North Island and western South Island will begin during Wednesday morning.

The low itself will peak in intensity on Tuesday and Wednesday out to sea, then weakens as it finally crosses New Zealand this Friday, falling apart by Friday evening leaving just a few showers only for northern NZ.