HUMANS weren't the only ones going hopping mad in the heat today.

Beachgoers were surprised to see a joey spring into the surf at Mordialloc as the mercury soared to 36C in Melbourne, its highest temperature since March.

media_camera Reader Loren Shallies sent in these photos of a joey at Mordialloc Beach

A southerly wind change brought cool relief after the temperature fell dramatically after 6pm.

The city is set for another scorcher tomorrow, hitting a forecast 30C, while Mildura will swelter under 39C heat.

A grassfire sparked by live firing activities on the Puckapanyal Army Base earlier today.

Defence contractors have brought the fire, 4km northwest of Puckapunyal, under control after the blaze went up in sweltering conditions just after 1pm.

GALLERY: Joey takes a dip during Melbourne scorcher

The Country Fire Authority has warned that smoke may be visible from nearby communities and roads.

It comes as the Bureau of Meteorology warns an afternoon wind change sweeping through could potentially complicate efforts to combat several bushfires across the state.

Forecaster Peter Blake said the mercury could still reach the forecast 36C in Melbourne before the strong southerly shaves up to a predicted 14C off the scorching weather from about 6pm.

"Initially it's going to punch through," Mr Blake said.

"If there's any fires around and you have a wind change it would potentially heighten the problem."

media_camera Locals Maddy, Remy, Issy, Claire, Ella and Sophie Lowrie soak up the sun at Brighton Beach. Picture: Jay Town

It took 14 firefighting crews to battle a grassfire at Mt Macedon this afternoon, which spread up to 8ha before it was brought under control.

Fires have also sparked at Echuca, Port Fairy, Rapunyup, Kellalac and Wurruk, in the state's east.

The wind change is set to bring a dose of cool relief for Melburnians labouring under the intense 33C heat.

"It will drop 10C quite easily, if not 10 to 14C. Aireys Inlet peaked at 30C and is now sitting at 25C," Mr Blake said.

But the change is unlikely to bring much of a reprieve in the state's far north, when it is likely to hit Mildura at about 10pm.

There is a total fire ban in the Mallee, with Mildura searing at 37C by 12pm.

media_camera Brighton Beach as the mercury soars. Picture: Jay Town

The baking conditions have also caused delays to regional rail services to Seymour and Bendigo, with V/Line implementing heat speed restrictions due to expanding railway lines.

If the mercury matches predictions it will be the hottest day since March 12, according to Bureau of Meteorology senior forecaster Scott Williams.

Temperatures will soar to 38C in Mildura, Swan Hill and Kerang. Bendigo, Wangaratta and Seymour will all hit 35C.

Mr Williams predicted this week would unleash a "roller-coaster" of weather with cold fronts expected to hit the city on Wednesday.

There will be a cool change this evening as a strong low pressure system blows in.

Up to 20mm of rain is expected tomorrow as temperatures drop to 20C.

A southerly blast will mean temperatures dropping to 17C on Thursday with forecasters predicting thunderstorms and hail for the city.