If negative PBT, but you believe they are impaired for a reason other than alcohol:
"As I have observed you driving a motor vehicle and in my opinion you may be impaired by a substance other than alcohol alone.
I now require to accompany me to a Police Station/Place for the purpose of a drug impairment assessment and to remain there until assessment has been carried out or until three hours after driving, which ever is sooner.
Are you prepared to accompany me?"