Melbourne's balmy start to spring has claimed an early heat record, with the city enjoying its warmest overnight minimum temperature for this time of year.

The mercury only dipped to 19.7 degrees by the 9am standard daily cut-off point, the highest minimum registered so early in the season in more than 150 years of records, said Bureau of Meteorology senior climatologist Blair Trewin.

Royal Botanic Gardens are in bloom. Credit:Ken Irwin

“A warm air mass and then clouds spreading in prevented overnight cooling,” Dr Trewin said.

While the temperature eased off after 9am, Thursday's overnight low was warm enough to beat any reading so early in the month.