Bloss, immediate thoughts are:
He's in a good negotiating position so I'd go for naming a reasonably high figure and let them beat him down (but not so high they rule him out). You can always go down in negotiations but you won't as easily be able to go up once you've named a figure. Presumably he wouldn't want to move for less than say, AUS$50k? So maybe add another, $5k? $6k on top for good measure but say that it all depends on other hours/travel/benefits/terms and condition etc and he'd have to know more before he can say for sure. They may just be after his current salary or they may ask this, so be prepared. He could say his current salary is $xyz and he won't move unless there's an improvement in that to the tune of $xyz. Can you check out an online salary survey for his equivalent job? Or call a local agency and ask what they'd say he's worth in their opinion? Then you can say "professional advice is that with my experience I'm worth something in the region of $xyz"
If they're not kosher it's all academic so I'd assume they are for the moment. Although I'd say it's unusual for a decent sized co to use hotmail. I'd proceed as if they are and see what happens. You could always ask for their site address when you reply to the email.
Good luck to your dh, let us know how he gets on.