Hey Danny
I've left my boat unattended quite a few times while camping on Moreton without any issues but I must say its always a relief to find it there in the morning. There are three issues to contend with, the current, the weather and the dickheads.
Current - I use a way big sand anchor and with a very long & heavy chain, the boat is going nowhere with that setup - I also protect the rope from the bowsprit to ensure it doesn't wear through if it got rough. Some guys double-anchor front and back but I prefer to have the boat spin around the bow into the current rather than have the current drag side onto the boat but don't anchor next to someone who is double-anchored! Another tip would be to anchor deep as I have come out at dead low to find only a few inches of water under the boat and in the chop bouncing on the sand.
Weather - you are looking for south easterly or very little wind, avoid anything westerly as even light westerly's chop it up badly.
Dickheads - just have to take your chances I guess, I don't leave anything of value in the boat and leave the anchor light on and a sleeping bag in the boat so it looks like someone is in there. Safest place is the wrecks and site 1 has the best view of the water, the downside is no camp fires are allowed so I prefer further up the island. Most boaties overnighting at the wrecks are good blokes who are unlikely to knock off anything but a few tenders have gone missing from the back of boats - stolen or come lose hard to say.
At the end of the day I do it because its a great place to go and overnighting extends your fishing time greatly so if something does happen just have to deal with it. Same goes for the car and trailer back at the ramp!
Cheers
Crunchy