Ms Lewis was last seen riding her bike from a friend's house on Stenlake Avenue in north Rockhampton at about 10.45pm on Saturday, January 14, 1989.
She was riding a short distance to her Alexandra Street home, but neither Ms Lewis nor her bike was ever found.
Queensland Police now strongly suspect she was murdered around the time she disappeared.
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 re-consider 33 years of silence and come forward to police."
This week is National Missing Persons Week.
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