I think Australorps make great meat birds, very roundy with meat on their bones (rather than some that are long and skinny). If you were doing Cornish X broilers you would slaughter them at 6-9 weeks, and if you were doing black broilers you would slaughter at 7-10 weeks, but I would not call...
Since an Australorp is a dual-purpose bird, not officially a meat bird, you can slaughter anytime. I prefer 16-20 weeks, as then the meat is still tender but flavorful and you have a good sized carcass (at these ages my Wyandottes were 3-5lbs dressed out). At 8 weeks an Australorp has barely hit...