Our small swimming club in Bundeena (Port Hacking/Cronulla Area) have noticed some Jimbles these past few weeks. Even though ,at the time, we couldn't spell them, poor Steve felt their sting. He said it was similar to a Blue Bottle but didn't last as long - I thought the welts on his arm were worse than a Blue Bottle and was happy that it wasn't me that was stung.
I have since seen them fairly often (I swim with goggle/snorkle) , one time there had to be about 50 - 75 in a 50 meter stretch of water. I somehow managed to weave my way through them without getting stung which proves that they don't attack and if you stay calm you can avoid them.
At the moment the sea is a relatively warm 23 degrees, and we have had plenty of rain and the water is a bit "green" - I believe all these factors are conducive to the "Jimble Invasion". Hopefully, they will soon move on so we can have our sea back again :-)