A huge swathe of Australia baked in mid-40 degree heat on Friday, with more records likely to be broken at the tail-end of a heatwave that set a slew of national highs last month.

The mercury was tipped to reach at least 45 degrees over a region stretching from northern Western Australia into Victoria and the NSW Riverina.

A view of the sun rise from Newport beach in Sydney's north on December 28, 2018 - the warmest night on record for Australia for that month. Credit:Nick Moir

Melbourne exceeded its predicted top of 42 degrees, reaching 42.6 degrees. Avalon, to the city's west, reached 45.8 degrees before a cool change knocked that down to under 25 degrees in less than an hour.

Walpeup in the state's north touched 46.6 degrees - not far shy of Victoria's January record of 47.2 degrees set in 1939 - while across the border in South Australia, Marree got to 47.2 degrees.