I agree that it is most likely in the family Crassulaceae. However there are a lot of South African stonecrops that are not Sedum, which I would imagine are garden plants in Australia.
Possibly Sedum rupestre. I think S. acre and S. album have fleshier more rounded leaves. However, a visit to a garden centre will probably show other possibilities. Are they likely to become a problem? There are some sedum patches along Isaacs Ridge near houses.
Thanks Betty, Don and Mike. Weeds of the SE lists it as naturalised but no mention of it being invasive. Regarded as a potential sleeper weed in the Environmental Weeds of Australia database https://keyserver.lucidcentral.org/weeds/data/media/Html/sedum_rupestre.htmp