Why? Its 4.1m AUD. Given the salary of devs and the scope of such a website, the original budget seemed pretty optimistic.
This was Accenture and Deloitte - not some backyard dev shop.
But the problem is with the assumption that the website needs a full overhaul. So often a full overhaul is where projects go to balloon in cost by 20x. An outside agency sells the leadership on a big picture full of fluff about "modernization" without any connection to real improvements.
A better approach would be to determine the most important weaknesses of the existing website, and incrementally improve them. But big organizations struggle with this. Government agencies are probably even worse than big corporations, but big corporations are terrible too.
(It was slightly weird that the old website didn't support https -- but on the other hand, I can't really think of a realistic case where that mattered. And I reckon they could have sorted it out for closer to $0m than $100m.)
It always did if you linked to https://reg.bom.gov.au , eg https://reg.bom.gov.au/products/IDN60801/IDN60801.94768.shtm... https://reg.bom.gov.au/products/IDR711.loop.shtml
Devs are shocked to hear that their 500k salary makes the project cost more than a 50k they think it is worth.
Jesus, people hear one tidbit about prestige staff engineer positions at Microsoft, Meta, and Google, and then assume we're all getting those numbers.