I have a hypothetical question. If someone contracts a virus at work is it in breach of the HSAW act?

I have done a thread on current risk assessment based on community exposure. From there you can adjust exposure rates to near zero by the time we return to work.

Not necessarily. If you are assessing the risks, putting in proportionate controls to the best of your knowledge, monitoring them etc then probably not. If you tell your worker to come in even though they tell you they're sick and then put them in close proximity to each other with no other protection, then probably yes. If the virus is ebola, the controls would have to be way more rigid than if it's the common cold.
The Act does not require absolute prevention of every possible consequence, hence 'so far as is reasonably practicable'

One thing to take into account, is that the restrictions will likely be lifted on a regional basis, so some sites will be open before others, and this may affect your supply chain risks, or put normally quiet sites under more strain than they usually have. Coming out of lockdown is potentially more troublesome than being in lockdown - the uncertainty returns.