It is indeed way over what Collingwood initially estimated.
The earliest instance of an estimate I can find is $65M in 2019/2020. The cost of the WTP then grew to approximately $120M during the design phase, but these cost increases were due to scope changes, including expanding capacity of the plant (they were aiming for an “as much as possible” approach, as the servicing needs of the area were growing faster than originally anticipated), upgrading the shorewall barriers, replacing more of the internal systems, and building in redundancies/capacity for avoiding full WTP shutdowns during repairs and for future expansion, as this is not going to be the final buildout of the plant. They are planning for a second phase in the future that will double capacity once again. I would argue that this cost increase actually makes sense since the WTP design itself changed significantly.
The second cost increase came in when the project went to tender, the lowest bid came in at $270M, over double their original cost estimate. It’s really hard to say why exactly the cost estimates and the lowest bid were so far off from each other. The official reasoning from the town is that it can be chalked up to the typical “supply chain shortages and construction cost inflation” issue. My guess is that there is probably an element of this here combined with the way the cost estimates were done in the first place. It is not uncommon for costs to escalate significantly if the initial cost estimates are taken at an early design milestone.