Hey mate.
The most famous sport multi is in the Blue Mountains near Sydney. There's a few others near Sydney, and a few in Queensland. Check thecrag.com for an online guide that is more used here locally.
18 (5.9+, 5.10- ish) is about the minimum, there might be enough for a short trip, but if you can train up this year to get more solid up to 5.10+ or even 5.11- you will have heaps of options.
Whymper, Dirty Rotten Pig, Schwing, I Don't Tip, The Rift (at Heathcliff), This Sporting Life, Weld Party, Mirrorball, Bunny Bucket Buttress (the most famous and popular), Randy Rabbit Ridge, Hotel California (using the variant start).
Also "Whale of a Time" on the Wollongong cliffs south of Sydney. And some very easy slab multis at Mt Gibraltar southwest of Sydney.
In Queensland look at Mt Tibrogargan. Caves Route, Troposphere, Sunburnt Buttress. Don't know much about that state's climbing sorry. It will already be getting hot up there in September.
Victoria and Tasmania are much more trad states. If you *are* a trad climber, Djurrite/Mt Arapiles is much better for low grades 5.0-5.9 in YDS. And shits over most of the climbing above for quality.