The brother of a New Zealand nurse who disappeared from Sydney in 1980 had no idea about his sister's secret life as a prostitute and heroin addict.

Marion Sandford, 23, wrote a letter to her brother, Peter Sandford, in January 1980 saying she had gone to meet friends and would be back at the suburban Cammeray home they shared within a week.

"I am not at all sure when I will be home but it should be within two days to one week at the latest I suppose! Met a couple of friends. See you later, love Marion," she wrote in a letter posted from Sydney's Central Railway Station and post-marked January 29, 1980.

But she vanished without a trace.

Mr Sandford said his sister was the highest achieving of four siblings.

However, she had been troubled when she became a nurse and subsequently left New Zealand to join him in Sydney and sort out her drug problems.

Mr Sandford had believed his sister's drug use was recreational.

"I didn't have an inkling of the depths of Marion's addiction," he told the Glebe Coroner's Court inquest into her presumed death.

"I found out about the prostitution after Marion was missing."

Asked on Tuesday by counsel assisting the coroner, Sophia Beckett, what he thought Marion and her boyfriend Warren Mills were doing out every night, Mr Sandford said he believed they were visiting Mr Mills' ill mother.

The siblings flew to Auckland for Christmas in December 1979, and Ms Sandford pledged to sort out her drug problems, pay back her debts to family and attend to court matters in Sydney.

Mr Mills, whom she dated from 1978, said Ms Sandford was approached by their North Shore drug dealer David Pierce to do a drug run to Malaysia.

"She was quite keen (to go)," Mr Mills told the inquest.

Mr Mills said a drug run to Malaysia took two to three weeks and he didn't believe Ms Sandford had ever got a passport.

He said her addiction increased to using heroin four times a day.

Mr Mills said she became a prostitute and worked the streets of Kings Cross to support the pair's $A200 to $A300 daily habit.

On one occasion, she was picked up by two men who forced her to take LSD and raped her, he said.