Friday could be the hottest December day in Victoria's recorded history if temperatures reach their forecast maximum and are averaged out across the state, Dr Blair Trewin from the Bureau of Meteorology said. "Some parts of the north-west may get close to records for any time of year," Dr Trewin said. "The duration of the heat in northern Victoria is also quite significant. Mildura is forecast to have three days in a row of 45 and above and that's only happened once before, in 1939." Victoria's hottest December day was 46.6 at Robinvale in 1976, while the hottest ever day was 48.8 at Hopetoun in February 2009. Chilling out at Williamstown beach on Tuesday. Credit:Jason South Emergency Management Commissioner Andrew Crisp said emergency services would be on elevated readiness this week and at Tier 3 - their highest level - on Friday.

He said 40 planes or helicopters were positioned throughout the state to deal with new bushfires or the worsening of five fires that have been burning in Gippsland and the north-east since the Code Red day on November 21. Loading "I know we are prepared, are you prepared?" he said at a press conference on Tuesday. He singled out residents in towns near the existing fires - Bruthen, Ensay, Buchan and Gelantipy - to be particularly vigilant. The Department of Transport urged commuters to be aware of possible changes to public transport on Wednesday.

"At this stage there are no planned service changes on the metropolitan train and tram networks, however, Metro Trains and Yarra Trams will closely monitor conditions throughout the day," it said in a statement on Tuesday afternoon. Chief Health Officer Dr Brett Sutton said health alerts would be issued for the Mallee, Northern Country and North Central weather districts beginning on Wednesday. The alerts will remain in place for the Mallee on Thursday and then extend to most of the state, including Melbourne, on Friday. "We've gone into this summer period with relatively cooler temperatures [and] our bodies won't be used to dealing with the heat, so, especially consecutive days of heat, mean we won't be properly acclimatised and the risk of heat illness is more likely," Dr Sutton said. The December Australian Seasonal Bushfire Outlook. Credit:Bushfire and Natural Hazard Cooperative Research Centre

He said symptoms of heat exhaustion could include a racing heartbeat, headache, nausea, vomiting and dizziness. "If untreated that can lead to heatstroke, and that untreated can be fatal in up to 80 per cent of cases," he said. "So it's a really significant threat especially to those who are vulnerable. That includes the very elderly, very young, pregnant women and people with pre-existing conditions - heart disease, lung disease, kidney disease. "The real key message is to look for one another, look out for those most at risk, check-in on neighbours, check-in on loved ones and make sure they're OK." Bureau forecaster Richard Russell said northern Victoria could expect temperatures in the low and mid-40s on Wednesday, while southern areas, with the exception of Gippsland coastal areas, could expect the high 30s and low 40s.