One of your biggest challenges here may be finding 5 business class seats on the same international flights. Are you willing to fly different routes or on different dates? There's been many a post here on FT about trying to find 1 or 2 tickets on international routes, so 5, on a very popular route such as Australia, could be a challenge.

If you don't have that flexibility, you might be better going the upgrade route instead and buying a coach ticket. Some airlines require certain fare classes in order to upgrade with miles, so that's something you would need to watch for. It can get pretty complicated.