Yeah, I heard that the sales of fish medications is highly restricted in the UK so it must be similar in Australia. Don't worry, usually they should only be in the salt for a week to 14 days max. But in his case, I think building up his slime coat to fight off the bacteria is more important.
If you can't find Furan-2, then go ahead and use the Melafix. I normally don't recommend the usage of it because it has that potential to harm the labyrinth organ if overdosed. But the key word there is "overdosed." Now that you know, you'll be very careful not to overdose so I feel comfortable advising you to use it if you need to. In his state, it's better to have something in the water to help him fight off infection than nothing.