Depending where you are and COVID rules a lot of venues will let you hire a room for limited outlay or in some cases free and then guests just buy their own food and drinks. Just need a few decorations.

Maybe use a friend’s home? And ask guests to bring a plate as well as byo alcohol. Most people don’t mind helping out

Local playgroups often rent out their halls quite inexpensively, and they often/usually have kitchens which allow you to do your own catering easily.

I also find BBQ food to be quite good on the budget too.

A park for a bbq/party is often a good way to have a large number of people attend an event. Might be a good idea to check on numbers too and any possible restrictions due to covid?

A park with BBQ facilities would be good. It’s out in the open so kids can go out and play at the play ground and sausages and stuff don’t cost much when bought in bulk. Coles has a massive pack of sausages for heaps cheap and there’s also Costco where you can even get a good sized cake.
If the weather isn’t looking great in your area then a bowling club would be a cheaper alternative and they usually have buffet food so people have a variety of things to choose from.

You could either have it at a local garden or play ground – BBQ tea BYO drinks and Salad/desert
Hire a hall and just have finger food – buy alcohol from supermarket (shop around for best prices)
or book a function room at a restaurant – people can buy own meals and drinks or put some money down on the bar.

For my hubby’s 40th last year ( before covid) we booked out a restaurant and everyone just paid for their own meals and drinks

We just had my husbands 40th at the local soccer club. They had an attached bar and grounds to hold up to 200 people. We paid $500 which went on to a bar tab and then it cost me $500 for food. It was brisket buns from a BBQ catering business and meat and cheese platters(coles) and ice cream sticks (coles)to feed 80 people. After the bar tab people bought their own. The soccer fields were great for the kids to run on.

A park would be good for that many people. Get everyone to bring something to help with the cost, fire up the BBQ and play cricket!

A casual get together at a local park and have a b.b.q which is affordable.Have alcohol but also let friends know to bring there own as well.

If you could afford to hire a hall that has seating already and ask people to bring a plate of food to share and BYO drinks instead of gifts. You could supply the cake?

I would look for a pub with a nice beer garden. Maybe they might close it off for a private party. Especially because they would profit from people buying drinks. They even might have a BBQ you could use.