So previously you were flying NZ to USA transiting Australia, but you are now just flying Australia to USA?

Then yes the Australia International Passenger Movement Charge of $55 applies plus some SYD charges (I think $29.65 at the moment)

There may be no repricing in the original currency of the ticket (ZAR?). However, if the ticket was actually priced in ZAR but you paid in GBP (as opposed to paying in ZAR with a GBP card), then I have known airlines to re-convert the original total amount paid in GBP to ZAR at the current IATA exchange rate, which could be an explanation for this high charge.

E.g. ticket was ZAR 100000, but you bought it in the UK when GBPZAR was 21, so you were quoted a fare of £4762 and paid in GBP. Now GBPZAR is 19, so the system converts your original payment to ZAR 90476, and it will want ZAR 9523 + the additional taxes and change fee.