I don't know how the quarantine process works in Australia, but I would venture a guess food is sparse to say the least. Here, a fish hasn't eaten in 4 days at least, then it gets to me and if it's on its way to the west coast, it's another day. If I see a fish in a shipment that's in distress, it stays here and I send an email to the customer that it was held over and will go out the following shipment. I have an 80% recovery rate and that's why people buy from me. I don't make the huge margins most of the others do, but my specimens are treated with respect and if that costs me a few dollars so be it.
Here's the way I treat fish like that. Get some veggie pellets, New Life Spectrum are a damn good pellet food. Get a clean plastic or glass cup and dip out about 10 ml tank water and add in 1ml Seachem Plus to it and let the pellets soften for about 10 minutes and try to target feed the one that's skinny. One of the issues with tangs is to try to repopulate the flora in their gut as soon as you can and get as many vitamins, minerals and calories in as well to get their fat stores back up. Look for pellets with the highest fat content because most sea creatures energy is fat derived. Acanthurus tangs will eat plankton and I will feed them that as well.