Australian police have offered a $123,000 reward for information on the disappearance of a New Zealand woman 30 years ago - a mystery that has been linked to another missing Kiwi and some of Australia's most notorious crime cases.

Linda Davie was last seen in Sydney in April 1980, after leaving a simple note for her boyfriend that she was going away for a few days.

Her Bay of Plenty-based brother yesterday described her disappearance and the past 30 years as "devastating".

"We always think of her," said Nigel Davie.

Knowing what had happened would bring relief to him and his brothers, Mr Davie said.

New South Wales police announced yesterday that the state Government had put up a A$100,000 ($123,000) reward for information leading to a conviction relating to the disappearance and presumed murder of Ms Davie.

She is the second New Zealand woman on whose case Australian authorities have posted a A$100,000 reward in recent months.

In January, a reward was offered for information on the disappearance of 24-year-old Marion Sandford, who was last seen in January 1980 and went missing in similar circumstances to Ms Davie's.

A reward was also posted yesterday for information on a third Australian cold case - that of Tanya Farrington, who went missing from her Sydney home in 1979. She was 14.

NSW Detective Sergeant Robert George responded to news reports that police were investigating possible links between the Davie and Farrington cases and some notorious Australian crimes and criminals including serial killer Ivan Milat and the "Mr Asia" drug syndicate.

"Any suggestion of those links is just speculation," Mr George said.

Sydney's Sun Herald reported that between 1978 and 1989, Milat - a serial killer convicted for seven murders - constantly moved around NSW as a road worker, including into Sydney's northern suburbs where the two Kiwi women and Tanya Farrington lived.

Ms Davie was originally from Whakatane. She was 22 when she was last seen.

She had travelled in Europe, having taken time off university study, and had spent about two years in Australia, where she worked as a waitress, when she disappeared.

Nigel Davie, an Edgecumbe kiwifruit grower aged 54, flew to Sydney for the police announcement. He said publicity on the reward might encourage a witness who had felt unable to come forward at the time to do so now.

"I just hope something comes out of this and we can find out what happened to Linda. It would be good for all the family," he told the Herald.

"It just nags at you - what happened, what happened, how she got on and what did happen to her. It would be a relief to know, even though it's good or bad," he said.

Mr Davie has two brothers, born after Linda, who are now in their late 40s. Their parents, Tom and Merle, died not knowing the fate of their daughter.

"Mum was due to go and see her - just after she disappeared."

When Ms Davie went missing, she was living with her Australian boyfriend Stephen Lavender in a house they shared with others in Wollstonecraft on Sydney's North Shore.

The police said that on the night before she disappeared, Ms Davie went with a friend to The Manzil Room club in Kings Cross, where two men tried to pick them up and later dropped the two women home.

The next day, she visited Mr Lavender in a Sydney hospital where he was a patient. That was the last day she was seen.

Mr Lavender became concerned when Ms Davie failed to visit him in hospital on April 7. Three days later, he received a letter believed to be from Ms Davie.

The Sun Herald reported she wrote: "I shall be gone for a few days until the middle of next week. See you then, get well quickly. I love you lots. Linda."

Mr Lavender became more concerned when, on his discharge from hospital on April 15, he returned home and found that all of Ms Davie's belongings were still in the house.

Detectives say they believe the two men who drove Ms Davie and her friend home from Kings Cross on April 5 may have information vital to the investigation.

Police want to hear from anyone who knew Ms Davie. They would also like to speak with witnesses who came forward initially when she was reported missing in 1980.

In 2002, Ms Davie's disappearance was investigated by police in connection with the disappearance of two other women - Ms Sandford, who was last seen in North Sydney, and Mary Wallace - last seen in Sydney's Crows Nest in September 1983.

At the time of Ms Sandford's disappearance, she was living with her brother Peter in suburban Cammeray on Sydney's North Shore.

She penned a note to her brother three days after he last saw her, saying she would be home soon.

"I am not sure when I will be home, but it should be within 2 days to 1 week at the latest I suppose," the note said. "Met a couple of friends. See you later."