It's the second wave of extreme heat so far this year, prompting fears of a renewed bushfire threat.

But police are warning that lives can also be put at risk from mixing booze, drugs and extreme heat.

"It is important for everyone to look out for themselves and for their mates," said Acting South West Metropolitan Region Commander, Acting Assistant Commissioner David Eardley.

"We strongly encourage revellers to be sun smart, stay hydrated by drinking plenty of water instead of alcohol, keep cool and have a plan in case you or a friend feels unwell."

Big Day Out will be held between 11am and midnight at the RAS Showground, Sydney Olympic Park.

Police said a high-visibility operation would also be at the sold-out event, targeting anti-social behaviour, alcohol-related crime and illegal drug use and supply.

"Recreational drugs are dangerous, illegal and potentially life-threatening, especially when combined with alcohol and in the predicted weather conditions," Mr Eardley said.

Last year, more than 300 people were searched, with 76 allegedly detected in possession of illicit drugs.

Forty-nine people were charged with drug possession, four with drug supply offences, and 16 were issued cannabis cautions.

Another 165 people were refused entry for being drunk. AAP