You might be in clearer waters there, but also cooler, so I'm not sure its any safer. There was a Qld Parks officer attacked approx 10km from there in April, which would've been an unprovoked attack.
Every man and his dog is spear fishing the northern GBR these days, its new craze, to the point where the selective taking of large fish is a real problem. Every weekend there will be hundreds of spearos along those reefs not in the green zone, this is the first attack in a while so the odds are still pretty low, but its been a big year for shark attacks. This guy owes his life to a few things going his way.
Granted its all perspective, and I've never dived Britomart, but Otter reef is clear and actually very shallow fringes. We didn't go the whole way around it but it was hard to find more than about 8m of water near any coral, the viz was much better than that. A few people might take a bit more notice of their fuel capacity and the tides though, the channel into Dungeness is one to understand clearly if your life is going to depend on it.