Reroute/Reissue after departure should be USD75.00 for :-: 1/2/3. It may be their own service charge or similar tacked on top if they're not an SK office but a GSA or suchlike.
SQ TG AC NZ and probably some others have ticket offices in SYD - go to one of them if SK still won't budge on the USD100.00 figure.