No.
As mentioned above open jaw tickets don’t attract any particular increase in charges… unless of course your departure is from somewhere like London where it attracts a luxury premium departure tax (but that’s nowhere near $2000).
What may have increased is the fuel surcharge, but again nowhere near the levels you were quoted. Against that is the strengthening Aussie dollar, which would reduce some fees very very slightly.
Without knowing the ins and outs of your ticket, difficult to say what the charge came up as it did.
The only thing I can think of is that they intended to bill you the new charges, and refund the old ones… and it shouldn’t be ‘in addition’.