Ok so heres the situation:
Center is active with base in Sydney. We fly YBBN - YSSY. Sydney airport is very close to a boundary of a FIR, now lets say u arrive from Brisbane, north of Sydney. When u get very close to Sydney u still find urself in a different airspace than the one that our Center operates in.
Do u tune into Center once ure 270nm out of Sydney but in a different FIR, or descend on ur own because there is no FIR in ur actual FIR?
Heres a pic for reference: