U need to get police certificate once visa office asks u. U can't get before tht, bcz thy r valid fr a certain time period only.
I suggest wat u do is tht u go to a police station in Saudi Arabia and ask thm process to get a police certificate for immigration purpose while u r leaving abroad. Ask thm wat things thy need and try to complete thm prior leaving, like finger prints, translation, attestation etc...
Bcz if u get a police certificate earlier thn it wil b futile, if u get a request later to submit it.
Also inform LVO tht u r moving to Australia, if u hv not yet done tht.