OK i've just realised why that wont work.

OK how bout just an even split then proportional to whats owed:

BTC_owed = 1768.9876932
AU_bitcoins_owed = Convert all AU amounts owed to BTC as of todays rate, and add them up

payback_ratio= ( BTC_owed + AU_bitcoins_owed )  / 1768.9876932

Howmuch_you_get_back = your_amount_owed * payback_ratio

BTC people should still be "up" even though they're getting less bitcoins than they put in, due to the rise in value.

A$ people will lose out a bit, but will still come close to breaking even.