Building ships requires workers, but not millions. More recently military ship building has been so haphazard, that many trades come up through civilian ship related industries or dual role industries. There is a limit to how many people are working in a shipyard directly working on building a ship.
The issue is many of these workers aren't located in the ship building area. There just isn't the ability to create so many workers in such density. Apprentices aren't just multiplying in water, So many would require relocation, from interstate and interstate relocation doesn't always stick and often is very difficult these days, also different top end management or low end apprentice.. Which is why doing blocks outside of the shipyard may be more useful than just trying to build a mega shipyard, bigger than we already have. Also having some distribution of the capability makes it more resilient as well. A fire, a blackout, pandemic, economic crisis, plus having options in other locations that are desirable is useful, your aren't just locked in both employee and employer geographically. People are more likely to relocate if they are already working on a project with a branch or a contractor on that project.
Many trades also tend to burn out. Your not building trades that are going to work for ASC for 40 years. It's not just trades, its semi professionals, which is something Australia and its AQF doesn't do very well. Advanced Diploma and Associate degree. Key people with experience, but also sit in between front line trades and Engineers and project management. I had an argument with a leader of the Attack class program about this stuff.
IMO I think WWII style conscription is unlikely, and in modern society, it wouldn't work. It only worked in WW2 because the female workforce basically came out of of the wings and picked up the slack. That isn't possible any more, they are already working. I don't think Australia can scale its military in the same way, warfare isn't the same, society isn't the same.
These days, machines are huge part of the fight. Drones, smart munitions etc have a huge outsized impact. Workforce needs to be prepared to produce that.
In a hot war, yes, we can build the hulls from steel, but will we be able to get all the other logistics, radars, VLS systems, guns, sensors, engines, gas turbines and the other millions of parts that make a warship.
We make almost nothing of that stuff anymore, and that we do, we are reliant on others. However, neither can pretty much anyone else. Also how many VLS and radars and gas turbines are you intending to develop, build and deploy in the actual war. Many of these are realistically going to be made from logistic stockpiles. Stockpiling 1000 CPUs, is a lot cheaper/faster than building a $60billion semiconductor fab factory. These could already be stockpiled and automatically refreshed and updated through existing retail/wholesale channels if we were clever.
The Japanese and the koreans do most of this themselves. We should definitely talk to them and the europeans about what Australia can do that is either a bottleneck or a area the global supply chain is fragile. The Japanese and the Koreans are actively trying to have out of their nation duplication of supply lines, because in a fight they may not be able to function, and certainly won't be able to scale.