I was about to point you out some used Cannondale MTB tandems for sale when i realized you're in Australia. Even so, you might check the major tandem dealers here in the US and see if they will work with you. Cannondale MTB tandems aren't rare, you ought to be able to find what you need for much less than the $4,000 you mentioned even including shipping from the US to you. Tandems East, Mt. Airy Bikes and Precision Tandems are the more well-known dealers that I'm familiar with. Other folks on this forum can probably add a few more. Tandems East might even have some new ones left, while Mt. Airy Bikes seems to have a few used ones if their web page is correct.