Welcome to the scam that is NBN.
Big Retail Service Providers (RSPs) do not buy enough backhaul to accommodate the amount of bandwidth that NBN connections demand partly because NBN is selling its capacity to RSPs at 17-20 dollars per MB. As Telstra does not own any of the infrastructure as part of the NBN they too have to purchase this capacity, and surprise they don't! What does this mean? It means that most NBN users experience hideous congestion during peak hours, with resulting latency and packet loss making even the basic internet task devolve if you were back in 1995 on dialup. Telstra will purposefully try to use subterfuge, stalling, and other techniques to not let you out of your contract so you can try small RSP that may not be so bottom line based meaning you may have a chance of not being congested. Other RSPs actually limit the bandwidth either through their own mechanisms during peak hours, set usage allowances during peak hours (not successful), or sell unlimited usage plans but on the proviso you will get shaped if you download too much and too fast during peak hours.
All this equates is that most NBN customers who have been on NBN for sometime will try to "Churn" and use other Retail Service Providers (there are 4 ports to the NBN, so your connection does not automatically cease if you try a different RSP for a month.) You just get the new RSP to connect to Port 2. What is the cost of this? Well most RSPs know that Churns are happening because of the appalling product the NBN is, and either will lock you into a contract like Telstra does, and make it extremely impossible for you to get out of it unless you go to the TIO (Telecommunications Industry Ombudsman) OR offer no contract but then charge up to 100 dollars in "set-up" costs. The reality is setting-up a new NBN service is nowhere near this cost, but its for RSPs to basically profit off a known faulty product.
If that technical talk is abstract think of it like this:
A new technology on the market allows sewer pipe that is 10 times bigger and can flush toilet waste 10 times faster. The government feels that toilet waste needs to be flushed faster as a national imperative, and sets up a company (SewerpipeCo) to build these bigger sewer pipes into every home, but then when the sewer pipe leaves your neighbourhood it allows private companies to connect (for a fee to SewerpipeCo) to these larger sewer pipes which then allows them to sell the total sewer system to you. So lets say private sewer company Wasteaway advertises that it can remove toilet waste up to 10 times faster for 30 dollars more than its previous offering. Even though SewerpipeCo is charging 5 times more per cubic meter of sewerage than Wasteaway was paying previously on the old system, why would Wasteaway build bigger sewer pipes to connect to SewerpipeCo? Well its probably not in their economic interest: most people would not notice that their sewerage is not being transported quicker, and the speeds they advertise are clearly labelled "up to 10 times faster" and if you read the fine print there are very little guarantees about speed. Whats even more profitable is to sell higher speed sewerage systems at a higher price point to consumers knowing that there is very little imperative to actually increase the size of their own pipes. The problem is that the government never thought about what a rational company would do in this case (why would a company incur costs when possibly a majority of their users never think about the speed their sewerage is swept away). A rational company would attempt to get as many people on their sewerage plan and have the highest market share knowing that a percentage of people would be unhappy with the speed of their sewerage and eventually lose them (but not before binding them to a 24 month contract). A company with 5000 people who don't care about their sewerage speeds is probably more profitable than a company that has 300 people who care (and these people who care may demand the company invest in bigger sewer pipes decreasing their profits further).
The government should have provided extremely tight performance parameters to each waste company to protect consumers, and probably should have priced the amount it charges to private sewer companies more in line with what the market could reasonably expect for such a service. The technology of NBN, nor its actual rollout is or ever was the disaster. It was the actual agreements that the government reached with RSPs, and NBNco that is the real disaster.