The Prime Minister in his statement updating the community on Coronavirus measures explicitly stated that construction sites are an essential service and will remain open. 

In reference to non-essential gatherings of greater than 100 people not being permitted from 20 March, the Prime Minister stated that: 

“This does not apply to essential activities such as public transportation facilities, medical and health care facilities, pharmacies, emergency service facilities, correctional facilities, youth justice centres or other places of custody, courts or tribunals, Parliaments, food markets, supermarkets and grocery stores, shopping centres, office buildings, factories, construction sites, and mining sites, where it is necessary for their normal operation (although other social distancing and hygiene practices may be required in these settings).”

The full statement can be found on the Prime Minister’s website https://www.pm.gov.au/media/update-coronavirus-measures