Drug & Alcohol policy for contractors v employees
To quote the act:
Duties of workers
While at work, a worker must—
(a) Take reasonable care for his or her own health and safety; and
(b) Take reasonable care that his or her acts or omissions do not adversely affect the health and safety of other persons; and
(c) Comply, as far as the worker is reasonably able, with any reasonable instruction that is given by the PCBU to allow the PCBU to comply with this Act or regulations; and
(d) Co-operate with any reasonable policy or procedure of the PCBU relating to health or safety at the workplace that has been notified to workers.
Therefore under section B. A yes/no question like 'Are you impaired by Fatigue, Alcohol or other drugs' would suffice.
Don't waste your money on testing. It doesn't improve worksite safety. Just look at statistical injury trends over the last 10 years. Mandatory drug testing in many workplaces and we still can't reduce injuries and deaths. Your better to spend time & effort in upskilling supervisors