I'm not surprised that his initial reaction has been to refuse.
The cost of a return flight from Oz is a big gift and it could be that he believes that 'courtesy dictates' that he should refuse such a generous gesture at least once or twice before accepting.
I suggest you wait a few days and then write something along the lines of how you're living in hope he'll reconsider as your db's wedding day won't be complete witthout his best mate by his side.
Say that his presence will make your db's day, that so many people are longing to see him again, and that you can't think of a better wedding present for your db and his bride to be as a kettle is not going to fill the gap that his absence will create.
Tell him that you can arrange for him pop out of the cake if he wants, but that you'll have great difficulty superimposing his face beside your db's in the wedding photos.
If that doesn't produce the desired response, attempt 3 is to tell him you know that, if the situation was reversed, he'd be the first to make a similar gesture and that he'd better get cracking on with his best man speech as you're not going to take no for an answer and will be booking his return ticket departing on (date) returning on (date) in the next few days.
Does he have relatives he can stay with while he's over here? When's the stag do? I'd suggest he comes over for at least a week before the big day, and stays at least 2 weeks in total - 3 or more if he can get time off work - as it's a bloody long flight.
If the above fails, and a phone conversation doesn't cut it, organise a big screen Skype session for the big day or ask him to record himself on dvd giving his congratulations and suitable pithy comments - maybe he can 'feature' in the speeches and the toast as a surprise for your db?