Before you embark on a possibly long and drawn out process, is it possible for you to get an Australian lawyer to send a letter to your former landlord enclosing the judgement of the Victorian Civil and Administrative Tribunal and saying you will commence an action for enforcement (collection of the judgement) unless they pay up? Often a lawyer’s letter is enough to scare people into doing the right thing.
It seems to me that your being overseas shouldn’t interfere with your ability to compose and swear an affidavit. Here’s some resources for you:
The form of affidavits required in Victoria:
https://www.justice.vic.gov.au/home/justice+system/legal+assistance/affidavits/
(note “Each court and tribunal has special rules about the format of an affidavit, so please check with the relevant court or tribunal if this form can be used for your matter.”)
The Tenants Union should be able to help with the wording of the affidavit.
Whilst the first website seems to suggest that affidavits can only be sworn in Victoria for use in Victoria, the EVIDENCE (MISCELLANEOUS PROVISIONS) ACT 1958 – SECT 125-
https://www.austlii.edu.au/au/legis/vic/consol_act/epa1958361/s125.html
seems to suggest that an affidavit that is properly sworn elsewhere will do.
But to be safe, probably best to get the affidavit sworn before an Australian justice of the peace, which you might find at the Australian Consulate-General in Hong Kong:
https://hongkong.china.embassy.gov.au/hkng/SRVAU_NtrSrv.html
Hopefully it will be a matter of issuing a summons to your former landlord and they will pay up immediately instead of being dragged through the court.