Ms Lewis was last seen riding her bike from a friend's house on Stenlake Avenue in north Rockhampton about 10.45pm on Saturday, January 14, 1989.
The 21-year-old was going to her Alexandra Street home about a kilometre away, but neither Ms Lewis nor her bike was ever found.
Queensland Police strongly suspect she was murdered around the time she disappeared.
The friend who was last to see Ms Lewis, Kerry Bartley, said it had been "33 very long years of wonder and lots of unanswered questions".
They had been friends since the age of 13 and spent their last evening together having tea, watching a video and making plans for the following night.
"Every year that passes becomes harder, not easier, because there are no answers," Ms Bartley told reporters on Tuesday.
Ms Lewis lived with her grandmother after her mother abandoned her at just a few weeks old, and was later fostered by a Rockhampton local.
Ms Bartley said her friend did not want to worry her foster mother by staying out late the night she went missing.
Retired detective Ann Gumley, who worked on the initial investigation, said Ms Lewis's family and friends deserve answers.
"I spent a lot of my time as a Rockhampton police officer searching for Michelle and I still remain committed to seeking justice for her," she said on Tuesday.
The $500,000 reward is for information that leads to the conviction of the person or persons responsible for Ms Lewis's suspected murder.
"We believe there are people out there with information," cold case investigator Detective Senior Sergeant Tara Kentwell said.
"Now is the time for those people to reconsider 33 years of silence and come forward to police."
Police have appealed in particular for a man who called Rockhampton Police Station about midday on February 18, 1989 to contact officers.
He terminated the call while being put through to detectives.
Sen Sgt Kentwell said a coroner in 1994 found no evidence implicating anyone who had been interviewed as part of the investigation into Ms Lewis's disappearance.
A 1999 review into crimes committed by serial killer Leonard Fraser found no information to suggest he was involved in Ms Lewis's murder, she added.
Fraser died in January 2007 while serving three indefinite jail terms for killings in Rockhampton between December 1998 and the following April.
This week is National Missing Persons Week.
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