Mwenenzi is correct, you should have no trouble accessing the Shuttle bus between International (T1) and Domestic (T3) in Sydney - and vice-versa, here is the relevant
Qantas Website Page.
In Perth, as you're flying SYD-PER-SYD, you will be able to remain airside. If the returning PER-SYD is not the same aircraft as that which operated SYD-PER, you'll just walk from your arrival gate to the departure gate; no need to re-clear security.
Regards,
BD