There is no set age. Time of year and how you feed and manage them could play a part. Heredity can factor in. Strain is probably a bigger part of that than breed. If the person selecting which get to breed uses early egg laying as a criteria then most will probably start pretty early. If it is not a criteria then they may start later. If they are breeding them for show, early laying is probably not a criteria. I don't know where you are getting them in Australia, you might be able to chat with the source and see what they say.
You have to have enough for averages to mean something. I don't know how many you are getting. Some start laying earlier than others. If you only have one or two, which one did you get, an early or late starter?
If I got 10 hatchery RIR pullets here in the States, I'd expect one or two to be laying by 20 weeks. I'd expect half of them to be laying around week 23 or 24. I'd expect practically all of them to be laying by 7 months, maybe a week or two earlier. But I would not be shocked if one waited for 9 months to start.
If I had a hundred hatchery RIR pullets here in the States I'd be a lot more confident in these proportions. If I got them from an individual breeder I would not know what to expect.