"The monthly temperature is 1.5-2C warmer than average for May overall."
The unseasonal weather is highlighted by a high of 24C tipped for Christchurch on Wednesday.
Aucklanders can expect to bask in a high of 22C today and the fine weather is expected to continue during the week.
Hamilton is expected to top 21C today and lower areas of the North Island, including Wellington, can expect highs of 19C.
Queenstown and Dunedin look set for light showers tomorrow but will still enjoy an unseasonal high of up to 20C.
Last week, the MetService said current weather patterns could lead to record-breaking temperatures in some regions and cities, including Invercargill.
Previous May highs included 24.6C in Auckland in 2011, 22.4C in Hamilton in 2004 and 27.3C in Christchurch in 1978.
In March this year New Zealand recorded its second hottest month ever - the nationwide average temperature was 19.5C, just below the record 19.6C in February 1998.