Try Celebrity Wigs at Bondi or Individual Wigs at Darlinghurst. Both could do mail order. I've had wigs from both places and many people didn't even know I was wearing a wig, they looked so good. After you buy it, go to a hairdresser and get them to lightly trim it to suit your head. That way it will look more natural. They tend to put a lot of hair on them to suit every head! Even if it's just the fringe and a bit on top, to fit your face and head shape!
Chris on this site, got one mail order from Melbourne. Ask her, or phone Jacaranda Lodge at the SAN hospital in Sydney for their number. Chris left brochures there.
All the best. I'm sure you'll find something. You can claim from your health fund too if you have Extras cover!