Contention ratios are poor guide to congestion
Contention ratios are a very poor guide to congestion. It is possible to have a 1000:1 contention ratio and for there to be no congestion, equally it is possible to have a 2:1 contention ratio and for congestion to occur. It all depends on what data is being sent on the network.
The speed tiers on the NBN actually contribute to congestion because the artificial caps on speed mean that large downloads (e.g. video streams / software updates) are queued rather than being downloaded as quickly as possible. To understand this, consider two RSP both with 100Mbps of CVC and 100 customers. The first has all on 12Mbps plans. The second has all on 1Gbps plans. The contention ratio is much worse for RSP 2. Consider the scenario where at 1 second intervals a new custom starts downloading a 50Mbp file (e.g. 100 seconds in, customer 100 starts downloading the file). Which RSP will have the higher congestion?
The simplest way to reduce congestion is to cap the speed or reduce the priority of packets for users downloading a high volume of data.
The quickest way to improve the global standing of Australia in internet rankings is to remove speed tiers on the NBN and have ACCC crack down on RSPs offering unlimited quotas. If speed tiers had not existed then FTTN would not have been justifiable.