Adelaide posts another 40C day

Adelaide has posted its seventh day above 40C in December, a record for the month.

Adelaide has posted its seventh day above 40C in December, a record for the month. (AAP)

Adelaide has closed out its hottest December on record with the seventh 40-degree day of the month.

The mercury hit 40.6C on Thursday ahead of a cooler change forecast for New Year's Day that should drop the temperature to the mid-30s.

That gave the city is seventh day above 40C after four consecutive days above that mark earlier in the month.

The previous record for December was four days above 40C in both 1897 and 1898, forecasting group Weatherzone said.

December also posted the second highest number of 40-degree days for any month, only behind the 10 days posted in January, 1908.

Weatherzone said El Nino conditions contributed to the record heat with frequent troughs moving in from the west and collecting heat from inland Australia.

The Bureau of Meteorology has also forecast continuing warm to hot conditions for the first week of January, with temperatures for Adelaide in the low to mid-30s.