Realistically there's nothing near the station food-wise, with the exception of a Coke machine and a snack machine within the waiting room at the station itself (make sure you have coins!)
Even though you may have a 24 minute wait, the process actually varies dramatically depending on how many people are on the bus(es). When the train arrives the buses will normally be lined up outside of the station (having been used to bring in people to catch the XPT when it turns around and heads south again). People generally file pretty much straight off the train and onto the buses. For the more popular routes (which basically just means the one to Brisbane) there will be multiple buses, and they will normally load them one at a time, and they will depart as soon the they are full. The last bus will normally not leave until the scheduled time, but obviously the earlier the bus you get on, the earlier you'll arrive in Brisbane!
It's also not uncommon for the inbound XPT to be at least a few minutes late (in fact, I don't think I've ever seen it actually be on time into Casino!), so your 24 minutes probably isn't a real 24 minutes...
Your best bet is probably just to either bring something with you, and/buy something from the buffet car on the train (keeping in mind that they often run out, and they close a while before they get into Casino)
The other option is to catch the night train, which is a train all the way to Brisbane, arriving early morning (exact time depends on whether it's during daylight savings or not) - or just fly which is normally going to work out cheaper!!