If you're looking for travel insurance to go overseas while working in Australia on a 457 I think most travel insurance do cover you. For instance I got my cover with www.scti.com.au which says
You and the other people to be insured are, either an Australian citizen or permanent resident with no visa restrictions, or a New Zealand citizen or permanent resident with no visa restrictions who resides in Australia, or on a work permit which allows for a continuous stay of at least two years in Australia;
They're pretty good value too, but there's a couple of travel insurance comparators out there to look at different options!