An Australian man was jailed for three years after helping an elderly friend kill himself by providing him with an animal anaesthetic he bought in Mexico.

Merin Nielsen, 50, was convicted by the Supreme Court in Brisbane of aiding the suicide of Frank Ward, 76, in June 2009.

While Australia's Northern Territory introduced the world's first voluntary euthanasia legislation in 1995, it was overturned by the federal government and assisted suicide remains a crime in the country.

However, prosecutions are rare.

The court heard that Nielsen travelled to Mexico to buy a fatal dose of the veterinary anaesthetic sedalphorte, also known as nembutal, which is not available in Australia, the Brisbane Courier-Mail reported.

Nielsen admitted travelling to Tijuana at Ward's request but said he changed his mind and did not purchase the drug, telling police he found on his return that Ward had obtained the anaesthetic from someone else.

However, Justice Jean Dalton said it was clear the jury rejected Nielsen's story.

Dalton questioned his motive, saying he was Aus$12,000 (US$12,800) in debt, had power of attorney over Ward's assets and was the beneficiary of his will.

Nielsen's barrister argued that his client had been like an adopted son to Ward, who had heart problems and prostate cancer and was a member of well-known euthanasia advocacy group Exit International.