That's the ISP problem not nbn. Backhaul is expensive from nbn so ISP scrimps on it.
Grat, do you know that on fttn, the node has only 2 (two) fibers active from the box to the exchange? (there's a third spare one)
When you have hundreds of people on it, it's feasible that you get congestions.
Having said that; Phil; try Aussie Broadband I've mentioned in the opening post. These guys don't seem to have big evening congestions, and you can test them for a month, free of charge.https://forums.whirlpool.net.au/forum-replies.cfm?t=2685107
(Internode is also pretty congestion free at peak hours, but I don't know of any free trial offers)