There were provisions in the restrictions to make/keep sites safe and/or running on min staff.. It’s not a lockdown if one of the biggest employers are still open having people move around the city..
There are tens of thousands of people classed as 'essential' workers during lockdown and rightly so. Supermarkets have to be staffed, as do service stations, petrochemical facilities, airports, public transport, military, transport & logistics, hospitals, emergency services, agriculture etc.
Tens of thousands more in our financial services and various administrative roles also have to continue working in order for us to function as a society but they get the added bonus of being able to do it from home.
Construction is one industry where you can't just flick a switch, shut down a whole site for a few weeks - or months- and not have there being costly repercussions further down the track. There's environmental damage to half-finished jobs to consider. When building materials and heavy plant equipment are ordered for big jobs, it's usually weeks or months in advance and when it arrives it often HAS to be installed or put into use because
there's nowhere to store it safely.
Then there are the knock-on effects of long-term delays in major infrastructure projects. In a years time when your big-ticket freeway expansion, cross-city tunnel or level-crossing removal project blows past it's scheduled completion date, people -voters- will still arc up about it regardless of whether there were legitimate reasons for the delay. Big public infrastructure projects can be make-or-break for election campaigns while your larger private sector projects tend to be run by fairly big campaign donors.
As for the work itself - most construction work tends to be in outdoor and well-ventilated environments and the people employed in it are a lot more accustomed to mandatory PPE requirements and Safe Work Practices than your average massage therapist or data-entry clerk.
Finally, if all that weren't enough to convince you and particularly as this lockdown is primarily a Victorian concern - good luck getting a mass unpaid stand-down past the CFMEU!