Here’s the drill in summary (note this is what should happen ideally, though it may not necessarily happen this way):
Assuming it is not possible to get the witness to court the prosecution will seek to have his evidence admitted by way of statement under the “hearsay” procedures as outlined by exdc. This is usually done in advance of the trial so that the prosecution can take a view whether to proceed if the ruling goes against them, though it may be made on trial day. If the Magistrates decide to allow the evidence the witness’s statement will be “read” to the court. If they disallow it, it will not be used at all.
If the trial goes ahead and the statement is allowed the magistrates can place what weight they think fit upon it and they will bear in mind that you have not had the opportunity to contest it by way of cross-examination in court. If it goes ahead and the statement is not allowed the prosecution case will be considerably weakened from what you have said.
It is most unlikely that the CPS will seek to have the witness attend if he is indeed in Australia. They will have to pay his expenses. There is no such thing as a “subpoena” to force this. A witness can be summonsed to attend if the magistrates agree, but they are most unlikely to issue a witness summons for somebody to attend from Australia. There are no facilities to use Skype of cctv or anything of that nature for a witness to give evidence from an unofficial remote location.