This is a condition called Muscular Necrosis.
See the sticky thread "Diseases and Diagnosis".
It's not necessarily caused by bacterial infection.
As per my post in the sticky, it's can be a condition caused by stress in it's environment.
Check your water parameters, and post it here for us to see if there is anything out of the ordinary.
Is this a new tank? I ask because I see this condition regularly in tanks that have not been cycled completely.
Heat can also be a factor.
If the temps in your tank is too hot, it can cause this symptom. But since the weather is cooler now, it's unlikely to be the heat. But I'm not in Adelaide, so I have no idea what temps you are getting there.
You don't necessarily need to restart the whole tank. But you will need to check your water parameters asap. Something is stressing it out.
Definitely don't add them back into the larger population, until it recovers completely.
Thanks for the plug Ineke