I don't think it's a Perth thing, I think you just have changed friend groups and haven't adjusted for it. (imo, just from what you've said)

Talking to each person asking what they're interested and when they're generally free will get them interested and invested,

Knowing who has depression or anxiety will help understand where people are coming from and who you should put more effort into getting excited or invested,

Organising lifts and directions to people who are notoriously flaky will help, but be ready to wait around for them to get ready,

I find asking people to bring something small gets them invested, but too big and they might be overwhelmed. Targeting it to what their interested in is good; get the vegetarians to bring something to your BBQ, have the party animals bring tequila or caronas to your taco / Mexican food night, &c.

Building up a group where everyone wants to see each other will help; start with six people, you'll be able to see who gets along, invite new people slowly and people will look forward to the next gathering.

[Not sure if I'm really off on this, I used to be really socially awkward and I figured this out slowly.]