I have not actually used this online software before so I cannot vouch for its accuracy or quality but according to its description it does map out SIGMETs for Australia.
Product Tour: Weather ? World Flight Planner
It is $10(CAD)/month. I have thought about it but I don't know if I really want to spend $120/year just for the benefit of the SIGMET charts because that would be the only feature I'd be using.
I too feel that SIGMET charts should be produced by either BoM or AsA and made available to us through NAIPS. Would reduce the amount of headaches during flight planning.