Construction started in 1970 and it's been running since 1979. That's nine years, with ancient tech and the GDR's limited resources. There's no reason to believe a similar project should take nine years today in Australia.
If anything, large infrastructure projects take longer now with more stringent worker safety, environmental and engineering requirements. The replacement cost of 1960s/70s/80s hydro generating stations (particularly their dams) is much higher than what they cost initially. It's no longer acceptable, for example, to have 20 people die on your dam construction project. It's also a hell of a lot easier to forcibly relocate local people in an authoritarian country like GDR than in Australia.
For hydro projects, you can't discount the long planning phase due to the huge impact of the flooding caused. For the Markersbach plant, they had to "relocate the inhabitants of the village of Obermittweida". That kind of decision takes years to get approval for.
For a battery plant, you can plop it down pretty much anywhere, it needs minimal planning.
