I've been a Telstra cable customer for over 15 years and this has never been a problem, until the last 3-6 months. Everyone is complaining about it. It is a Telstra infrastructure or design problem, which they need to fix ASAP.
fwiw, my public IP address is in a subnet with 16000 other computers now (255.255.192.0), I do not remember it being this large in the past, and it doesn't surprise me there is congestion on my subnet if there are another 16000 computers in it!!