Temperatures this morning were the coldest of the year for parts of the country with even some people reporting their first frost of the year.

WeatherWatch.co.nz says this is very late in the season.

For others the frosts have returned after a long stretch of milder than usual weather, says a spokesperson for the weather ogranisation.

The shortest day of the year is also historically the beginning of the coldest period of weather for the next six weeks, says WeatherWatch.co.nz.

"The winter solstice is a precise moment in time - this year that moment is 9:13pm on June 21. With the solstice being so late in the day it means that both June 21 and June 22 are the shortest days of the year.

"Tonight, colder air moves northwards with frosts expected as far north as Waikato, even potentially around the Bombay Hills (where it's borderline due to a south east breeze still blowing through).

"Hamilton is forecast to drop down to +1C tonight while southern parts of Auckland (like Pukekohe) have an overnight low of +3 (and downtown Auckland drops to +6C). Frosts in Waikato may be a bit patchy."

WeatherWatch.co.nz says frosts will be widespread across inland parts of the South Island tonight as powerful high pressure slowly moves in with clearer skies, lighter winds - and a gentle southerly flow.

"Coldest weather will be over the South Island tonight with the Frost Forecaster at RuralWeather.co.nz showing heavy frosts across a number of regions and temperatures inland dropping below -4C.

"Heaviest frosts tonight will be through inland parts of the South Island."