I had some advice from FT and while in HKG was able to purchase for HKD98 a pre-paid
"Peoples" IDD Sim card.
In fact, this weekend I purchased another so we could contact each other or make calls at a reasonable cost while we were there. Both came with "bonus" HKD30 'refills'.
How reasonable? Try HKD0.25 per minute anywhere within HKG and elsewhere including the LOTFAP and any Oz landline. It is HKD2.25 to any Oz Mobile. (In Oz dollars that's AUD0.04 and AUD0.36 respectively)
So your HKG98 (AUD18) Stored value SIM can get you well over 8 hours of calls.