The extreme heat policy is determined by thresholds set by the Wet Bulb Globe Temperature - an international model for estimating heat stress levels taking into account air and on-court temperatures, humidity, wind speed and other factors.

Tournament officials will also make use of a dedicated weather station solely focused on conditions at Melbourne Park.

Tournament referee Wayne McKewan said a combination of extremes posed the greatest risk to the year-opening grand slam.

"When it's high humidity and high temperature, that's when it starts to affect the players, staff and patrons," Mr McKewan said. "We've got a heat policy at the AO that takes into account a variety of temperatures."

Tournament doctor Tim Wood was quick to dismiss the threat to players, though the heat has pushed seasoned pros to their limit before.

"Tennis as a sport is a relatively low-risk sport for playing in these conditions," Dr Wood said. "The time the ball is in play in total time for the match is relatively small. The amount of heat they produce from muscles exercising is relatively small in terms of what someone continuously exercising will do.

"The players have had plenty of time to acclimatise."

Temperatures in Melbourne are forecast to stay above 40C until Friday, with today's predicted 43C the highest for the week.

On-court temperatures, while not significantly higher, are expected to make life a little tougher on the players. "Players may feel very hot in their ankles and their feet through their shoes from the court surface," Dr Wood said.

The heatwave has the rest of the state on high alert, with the state government and fire services both issuing warnings of the severe conditions.

Health Minister David Davis stressed the importance of residents being aware of ways to combat the conditions expected over the coming week.

"It is important for all Victorians to prepare for hot weather so that they know how to stay healthy in the heat," Mr Davis said.

"We know that heat-related illness and deaths can occur when the 24-hour average temperature goes above 30C."

Fire Services Commissioner Craig Lapsley said the week's conditions would dry out the remaining green moisture in the state, making it a crucial time for emergency services.

"It is the turning point for the summer period," Mr Lapsley said. "This type of temperature, day after day, sees us bake in Victoria.

"Heat like this will challenge those things and we should think if there is a (power) outage of some kind, what it would mean to me, to us, and how we operate."

A total fire ban has been declared for today in Victoria in all regions except for the North East and East Gippsland.