The latest investigation, by Victoria Police, concerns the death of a 55-year-old Geelong man who allegedly killed himself using a do-it-yourself kit bought though a company affiliated with Exit International, the pro-euthanasia organisation founded by Dr Nitschke.

All of the deaths being investigated involved the use of the two suicide methods promoted by Dr Nitschke, the lethal drug, Nembutal or a nitrogen inhalant device.

A number of coronial inquests are already under way into the deaths.

A Fairfax investigation has found that Victoria Police have obtained a warrant to search the Telstra records of the Geelong man, Ross Currie, prior to his death in the remote Otway National Park on May 25.

Police believe that emails between Mr Currie and Exit International, Dr Nitschke and Mad Dog Brewing, the company which markets the nitrogen inhalent equipment, will provide some insight into Mr Currie's mental state prior to his death.

It can also be revealed that Dr Nitschke faces expulsion by the Australian Medical Association when its Northern Territory branch Council meets in November, after a move to suspend him last month failed after an error in the paperwork.

Dr Nitschke, who was suspended by the Medical Board of Australia in July, said "attacks" on his character were "coming from everywhere".

"I have received a smattering of letters of support from doctors, some very senior but watching this avalanche rain down, I don't know if I'm going to survive this," he told The Sunday Age.

The decision to suspend Dr Nitschke using the board's emergency powers to  "protect  public health and safety" came after he admitted in an interview with the ABC that he had supported a 45-year-old Perth man, Nigel Brayley, in his decision to commit suicide, despite knowing the man was not terminally ill.

The AMA has cited the same "adverse event", saying Dr Nitschke's "professional behaviour … was not consistent with the high professional and ethical standards for the Australian medical profession promoted by the AMA."

In other developments, documents obtained by The Sunday Age reveal there are currently five separate medical board investigations, one dating as far back as 2011, into Dr Nitschke's conduct.

The complainants are:

The Therapeutic Goods Administration's principal medical adviser, Dr Megan Keaney, alleged Dr Nitschke attempted to import the banned euthanasia drug Nembutal into Australia;

An anti-euthanasia advocate, Paul Russell, alleged Dr Nitschke had developed and marketed a nitrogen delivery system for the sole purpose of assisting suicide;

A Melbourne woman, Judith Taylor, who complained to the board after her 26-year-old son, Lucas, committed suicide using Nembutal after buying Dr Nitschke's euthanasia book, The Peaceful Pill Handbook. She is understood to have claimed that an online forum curated by Exit International encouraged her son to take his life;

A West Australian pain specialist, Dr Mark Schutze, lodged a complaint against Dr Nitschke after he addressed a meeting of medical and nursing staff earlier this year at Perth's Charles Gairdner Hospital;

The board, in its most recent – and ongoing – investigation, which led to Nitschke's suspension, concerns the circumstances surrounding Nigel Bayley's death as well as Dr Nitschke's general advocacy for the rights of people to commit suicide even if not terminally ill.  

Dr Nitschke has won the first round in his legal fight against the suspension, arguing successfully to have his appeal this November moved from South Australia to the Northern Territory.

The hearing, before a five-member panel, is set down for five days and is shaping up to be a test case on the idea of "rational suicide", whereby a person does not have to be severely depressed to make the decision to kill themselves.

It is rare for the board to use its special emergency powers to suspend a health practitioner. It is believed that there are only 11 reported cases where this power has been used. And, in all but one, the issue concerned doctors acting inappropriately with their patients, for example in cases of sexual assault, drug use or lack of surgical skill.

A spokeswoman for the board said it could not comment on "ongoing matters, as it will not compromise patient safety or the integrity of current investigations."

The Victorian Coroner would notcomment on how many Exit-related investigations are under way, but it is understood that in 2012 – the last public reporting period – there were eight assisted suicide inquiries, and Fairfax Media is aware of at least four more Exit-related deaths in Victoria being investigated now.

Support is available for anyone who may be distressed by calling SANE Helpline 1800 18 7263; Lifeline 131 114; Salvo Crisis Line (02) 8736 3295; beyondblue 1300 22 46 36.