Inside/outside party at your house? That way, people can get relief from the heat, but also it's focused more outside. Maybe add some water based activities? A water table, anything that can be slightly wet, but could also be taken further for those who want to get wet for relief?
Have you looked up nearby splash pads or pools to see if that’s a possibility? If you have enough space in your yard things like a shade cloth, gazebo (Kmart has options, I’m assuming you’re in Australia), pedestal fans and a sprinkler would make it more bearable. Shifting the party to earlier in the day will also beat the worst of the heat if that’s an option. We were at a park party recently and it hit about 34 and while it was stinking hot, with cold drinks and shade we all survived lol.
Do you have a church or community center nearby? They often have rooms to rent for free or cheap.