A weather shift has brought relief to revellers at the Big Day Out with dark clouds and drizzle taking the edge off temperatures that earlier peaked at 45 degrees.

The music festival in Sydney's west has been baking in stifling heat for most the day with fans struggling to deal with the record heat surge.

St John's Ambulance treated more than 200 people throughout the day, mainly for dehydration. A first aid room was at times packed to capacity with queues stretching outside. Inside, people were connected to saline drips or cooling down under foil blankets.

But a rain shower at 4pm (AEDT) went some way to cool down music-lovers who have been taking refuge under water sprinklers for much of the afternoon.

Others hid under shade or in rare spots of air conditioning as a usually fevered festival atmosphere crept to its slowest pace in years.

Bands including Band Of Horses and Vampire Weekend who drew smaller crowds than expected took time to warn the crowd to stay safe.

Jessie Ludlow, 20, was separated from her friends after a dizzy spell caused her to seek medical attention at the first aid room.

"It's just too hot to do anything but I didn't want to miss watching Grinspoon," she said.

"I feel better now after chilling out and drinking loads of water but I've no idea how I'm going to find my friends and I'm definitely not going back into the crowd."

Joshua Cox, from Maitland, was one of the lucky ones staying in a nearby hotel.

Spotted leaving the festival gates covered in sweat, he said he had no intention to miss the fun.

"I'm just going back to my room to put on another t-shirt," he said.

"As long as you drink loads of water and take it relatively easy it's not so bad."