The trickiest part of the $8k budget is definitely feeding everyone. With $4000 (according to my budget breakdown) that comes to $40 per head, including drinks. One certainly can cater one's own wedding, but I know it would do my head in. Plus it feels like cheating given that I'm trying to prove how far the money can stretch.
So I pretty much just googled caterers in WA for a bit and found that, for my town hall wedding, one can get a catered spit-roast with two kinds of meat and loads of different salad and veges for $18 per person. This was certainly down at the cheaper end of the range of prices I found, but there were a few caterers playing with this number, and it certainly wasn't the lowest price I found. Sure, it isn't the most glam option, but it's good hearty food that will fill everyone up, and it certainly has a homey, retro feel.
For the backyard wedding? What's more casual and Italian (and delicious) than woodfired pizza?
Cicciobella are a Perth-based mobile pizza oven company that offer three courses (calzone, all you can eat pizza, and dessert pizza) for a smidge over $20 per person. Brilliant!
Either options leaves you with $10 - 20 per person for drinks (depending on how much wiggle room you want to leave for contingencies). Counting one bottle of wine per person (go see the Dan Murphy's drinks calculator for a better estimate), and assuming you buy at LiquorLand or Vintage Cellars and get 30% off by buying by the dozen, you will be serving up some not shabby drinks!
Tip: Got rellies flying in from overseas for the wedding? Ask them to bring in their duty-free allowance in vodka and make up a big fruit punch for your town hall wedding, or mix it up with some red wine for sangria for your backyard (Spanish-)Italian do.