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Two series of tests done at the polytechnic in the last two years had both involved automotive engineering students, health and safety manager Andy Westgate said.
Before this year's drug test, the polytechnic was ''given some information that alerted myself into believing that there was a risk to an individual or group of students'', he said.
Mr Westgate declined to say whether any of the tests conducted this year returned a ''non-negative'' result.
The incident had ''prompted us to get a policy rewritten for the safety-sensitive areas'', he said.
At present, the polytechnic has a drug and alcohol testing policy only for its arboricultural students.
The proposed policy is under consultation with students and staff.
Luke Baines, an electrical engineering student and member of the student council, said expanding the drug and alcohol testing was ''a good idea''.
''Especially for the high-risk areas of the polytech; for people that are in the engineering department and other areas that are high-risk, like using a kiln.''
Mr Westgate said the proposed policy would allow for random testing if staff had reason to believe someone was under the influence of alcohol or drugs - and post-accident tests.
Otago Polytechnic's proposed policy steps were in line with other polytechnics and employers who hired their students, he said.
The Wellington Institute of Technology and the Christchurch Polytechnic Institute of Technology both confirmed they did not test students or staff for drugs or alcohol, although they did have policies regarding alcohol and drug consumption.
Tai Poutini Polytechnic, on the West Coast, undertakes drug and alcohol tests on some students, in a small number of programmes.
Tertiary Education Union president Sandra Grey said Otago Polytechnic's proposed policy would cover students or staff in all programmes, except for business and IT.
''Under what they've called safety sensitive areas, they have included a huge range of programmes,'' she said.
Mr Westgate said the affected programmes would be determined through consultation.
''The policy covers any programme where being under the influence of drugs or alcohol could be a problem, which could be through disrupting others' learning,'' he said.
''In fact, a fairly wide range of Otago Polytechnic programmes could fall into this category, as they may include machinery, equipment or decision-making responsibilities, and therefore need to be considered through this lens.''
Ms Grey said clear reasons were needed why an area had a particularly high degree of risk and why it should be included in testing.The proposed policy ''would cover everything from engineering to arts to farm school to hospitality to midwifery to design'', Ms Grey said.
She said the random tests would be ''highly invasive'' but appreciated the polytechnic was consulting students and staff before the policy came in.
Otago Polytechnic Student Association president Stephen Kohey said the association could not comment on the policy ''until we have had a chance to consult with the student body and our advisory board''.
In the past, the association had found it ''appropriate'' for students operating dangerous machinery to be drug tested, he said.
''OPSA are not OK with randomly drug testing students without a genuine safety concern.''
An art student, who declined to be named, thought a policy allowing drug testing in the art school would be ''humiliating and demoralising''.
''I feel it would violate the privacy of students and [create] an atmosphere of `guilty until proven innocent'.
''Potentially dangerous equipment is only used by a fraction of [arts] students very rarely, and always under supervision.''
Mr Westgate did not agree random drug tests were ''highly invasive''.
Consultations would finish at the end of May, he said.
Barring any changes, the proposed policy would be implemented by the policy committee soon afterwards.