A Northland doctor who was justifiably sacked for prescribing controlled drugs to her partner was a recreational user of methamphetamine and cannabis, it has been revealed.

Dr Lynda Marie Emmerson has lost an appeal in the High Court against the decision of the Medical Council's Professional Conduct Committee (PCC) that recommended her registration be cancelled.

The Health Practitioners' Disciplinary Tribunal subsequently cancelled her registration.

Dr Emmerson was working at Whangarei Hospital's Tumanako mental health unit as a psychiatric registrar when she prescribed controlled drugs— including morphine, diazepam, dihydrocodeine tartrate, amoxicillin and tramadol— to her partner, who was not a patient in the unit.

She also wrote prescriptions for her partner's mother and a work colleague.

Her employer, the Northland District Health Board, complained to the council about her conduct and the matter was referred by the council to the PCC.

The DHB recorded nursing staff as saying Dr Emmerson had told them she would, if tested, fail a urine test for abuse of different substances.

The PCC engaged another doctor to establish if Dr Emmerson had a substance-related disorder or a mental health issue.

She told the doctor that from 2013, she smoked small amounts of meth at four or six weekly intervals, but only on weekends, and last used it during Easter in 2015.

The drug, she said, boosted her energy levels, and she used it only when she had four days off work. Dr Emmerson said she also used cannabis, but would stop if she returned to work.

She tested negative for meth but positive for cannabis in a drug test in June 2015.

After the PCC's investigation, the tribunal said Dr Emmerson had not only "demonstrated an acute lack of judgment" but also "an irresponsible attitude to complying with her professional obligations, the criminal law and the duty of candour which she owes to her professional organisation".

The tribunal said her conduct jeopardised the safety of patients to whom she had prescribed drugs, patients in her ward and her colleagues.

"Dr Emmerson presented an ongoing risk to the public and profession. In short ... she is not safe," the tribunal said.

In her appeal, Dr Emmerson's lawyer, Chris Muston, said the PCC should not have laid charges because the complaints were the product of long-standing hostility against her from her supervisor and another senior doctor.

But Justice Mathew Downs said the tribunal was correct to cancel her registration and the use of meth and cannabis were not the only factors that contributed to that outcome.

"Dr Emmerson prescribed drugs of dependence in the same period as she repeatedly used both cannabis and methamphetamine," Justice Downs said.

She has another case awaiting a hearing date in the Employment Court. Dr Emmerson is appealing an Employment Relations Authority decision that ruled she was not unjustifiably dismissed.