Hi and welcome. UK here but have friends in Sydney with similar problems.
It must be hard, we can just walk into a shop here and buy them.
My 1st piece of advice would be to learn to clone so that once you do get seeds, they will last forever.
It doesn't have to be weed. Practise with clematis or tomatoes or something.
Once you have the hang of cloning, you have the potential to grow tonnes of weed from a single seed.