At Perth airport, passengers connecting between two international flights can usually stay in the transit area without going through immigration. If you connect from an international to a domestic flight, you enter the country here: you go through immigration and collect and re-check your bags.
The airport has 2 terminals or concourses (T1/T2 precinct, T3/T4 precinct). They are not connected after security: changing means going through security again.
Connecting passengers go through a transfer security screening.