Whilst in NSW, there are three limits 0.000* (Unlicenced drivers, Learners, P Platers and persons subject to a court order). 0.020* (Drivers transporting dangerous goods, driving more than a 13.9 tonne vehicle, drivers of ambulances, taxis, buses or any other form of public transport vehicle) and the popular 0.050* (all normally licenced drivers or riders) to operate a motor vehicle there are a variety of rules in a workplace.
Effectively, your employer (the PCBU) has a responsibility to ensure a safe workplace. This may involve implementing drug and alcohol testing. Whilst most of my clients set a 0.000 limit for alcohol, a few employers may set different limits. For instance I am not aware of any specific law that prohibits bar staff or security staff from consuming alcohol and ADF personnel on base are not subject to the laws relating to driving with a BAC >0.05 when on bases, so if an employer wanted to set an independent limit (and accept the liability for doing so) they could. Organisations like the ADF have set the limit for on duty personnel to be 0.02*.
If an employer sets a legal limit, that limit can be enforced as readily as any other rule in the workplace. If an employer set a Breath Alcohol Concentration of 0.005, then up until 0.004 you are permitted to be at and remain at work.