Internet has been doing fine for ages, id say almost a year or more after i switched from telstra due to their slow interent speeds. Got Optus and all was good. My download is currently 1.06mbps and upload is 0.36mbps. When i first got Optus it was around 30mbps on the speed test, while download speed was a little lower it was not much lower. What would of caused this fall in speed.
Check if there is an outage in your area
I am also in Brisbane, did a speed test at 10:45pm, normal speed for me.
cable speed can be affected by your local traffic. When Netflix just came out and optus having free Netflix for 3 month and non counted data, my speed test dropped from around 70 to 3.
Maybe everyone in your node is getting the latest episode of GOT