Ms Sidon was a single mother who reportedly lived a lonely life with her son in a housing commission unit at Ashmore.
She went missing in June 2009 but it took more than a year for anyone to report her missing.
Her elderly father in New Zealand contacted his local police, concerned that he had not heard from her. They then passed the information on to Gold Coast detectives.
At the time Ms Sidon's history of anorexia, anxiety and depression led police to believe she had committed suicide.
But new information now has her disappearance being treated as suspicious.
Southeast police regional crime co-ordinator Detective Inspector Dave Hutchinson said there has been no proof of life since June 21, 2009.
He said the ongoing investigation has led to the strong suspicion Ms Sidon, a part-time cleaner, was murdered.
"The pace of the investigation increased recently," Mr Hutchinson said.
"We have been able to confirm that she has not left Australia, she has not gone back to New Zealand and all inquiries would suggest that she has not left the Gold Coast either.
"As a consequence of our inquiries, we believe that she has met with foul play."
Mr Hutchinson said police have several persons of interest in the investigation, including Ms Sidon's now 28-year-old son.
A search is underway near Pine Creek Road in Numinbah.
Cadaver dogs last week located several "locations of interest" which police were now excavating.
Police are urging any acquaintances of Ms Sidon to make contact with them.
Her son still lives on the Gold Coast.
The Gold Coast Bulletin reported that Ms Sion's parents had both died in the years following her disappearance but she had siblings who still lived in New Zealand.
"Her sisters have been to the Gold Coast to assist with inquiries but they are not here now," he said.
Mr Hutchinson told the newspaper that Ms Sidon kept to herself mainly but did have some friends and did work as a casual cleaner.
- AAP
