The Irukandji is only 2cm in diameter and their sting is moderately painful with minimal scarring, however, approximately 30 minutes later the patient will develop severe back and abdominal pain, limb or joint pain, nausea, vomiting, sweating and agitation. In both instances medical treatment should be sought immediately.
To overcome the problem and allow people to swim safely, many beaches have installed stinger nets which help to keep the stingers out. Most also have a lifeguard on duty as an extra protection and are there to offer assistance should an incident occur.
It's only May, and even though I haven't been for a swim, I've had a couple of long walks along the beach in the water and felt absolutely safe doing so. Listening to the locals is the best way to find out what's happening in the area.