Thanks for all the replies everyone - it's good to get a fresh perspective on it. Unfortunately I can't inform the hotels prior to arrival to get special meals arranged, as my company books and I'm more often than not replacing another colleague at the very last minute (so the rooms aren't usually in my name when I get there).
I guess the main problem is I'm usually at a place for only 10-12 hours (e.g. I'll get there at 9pm and leave at 6 or 7am). In places like Darwin or Perth, there isn't much open at these times to get food, especially on a Sunday (even Woolworths shuts ridiculously early). If I arrive at normal hours, vegan food (or as 'vegan' as I'm going to get - probably not 100% but the best I can do) isn't too hard to find at all. It's arriving at unusual times that's the main problem.
I've started carrying canned '4 bean mix' which come with a vinegar type sauce. I add them to salads I buy if they're not substantial enough, or can have the beans by themselves if I'm desperate and can't get anything else. There aren't 'veganisable' vegetarian options at all the hotels I stay in, but at some I can order a vegetarian pizza without the cheese, or a risotto without cream or cheese. Steamed greens have been another option, but it's amazing how many places cover them in butter or cream (learned my lesson there).
I just had a week and a half off work and couldn't stop cooking! Made so many amazing, healthy, substantial vegan meals and felt so much better for it. Went back to work and didn't feel so great again

I'm sure it'll get easier as I get more inventive with it.