The disappearance is now a Major Crime investigation, which focuses on homicide cases, because the circumstances of his disappearance "were considered suspicious", police said today.
Mr Cairney, 31, left his Carey St home about 10pm on June 17 to catch a bus to the Lyell McEwin Hospital, Elizabeth Vale, to meet a friend, Detective Superintendent Grant Moyle said.
"When he left home he had little or no money on him and he has not accessed his bank account since that time, therefore the circumstances under which he has gone missing are suspicious," said Supt Moyle.
"We've made enquiries with the bus companies and we believe he has not got on the bus."
Mr Cairney has a fair complexion, is about 175cm tall and weighs 95kg. He has a medium build, a round-shaped face, receding hair and tattoos on his left leg and right arm.
When last seen, he was wearing a t-shirt, black vest, black trackpants, sports shoes and was carrying a green backpack.
Anybody with information on Mr Cairney's disappearance should call Bank SA Crime Stoppers ph: 1800 333 000 or online at www.sa.crimestoppers.com.au.