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Hi Bobe,
If you are flying both Qantas (or Jetstar) then I believe you won't have to change terminals to access the Qantas lounge (Int terminal Perth, Terminal 1 at Changi, Singapore). If the PER/SIN Qantas flights do cease on 21st July then your return flight may be affected. Qantas had indicated they were contacting affected persons progressively so may not have got to your booking yet.
If I try a dummy booking with your dates the booking engine shows no Qantas flight out of Singapore direct to PER on the 30th July, only Jetstar (JQ) or Jetstar Asia (3K). You may wish to confirm with Qantas directly as to whether your original return flight is canx.
Cheers.