FWIW, I was in Melbourne with a 'bag' emergency, ie. couldn't get them to stick and ran out on a Sunday. Long story short, couldn't get them anywhere so rang the gastro unit at Royal Melbourne. They said just come in and gave me a box of (concave) bags which worked a treat. As noted, they also have a 24 hour emergency department (parking is expensive though)...
You could ring them up and get the number of the gastro resident on call. I have that for RPAH in Sydney (I'm in Canberra and just had reversal Wednesday) so needed a number in case the proverbial hit the fan over the weekend. I'm sure RMH would have a similar on-call service.
