The woman known as JC testified in the NSW Supreme Court on Friday that Dawson told her his wife was never coming back after she had disappeared.
She said Dawson, 73, begged her to move into his house in early January 1982 because his wife had gone off with a religious sect and he needed her help to look after the couple's two children.
JC moved into the Bayview, Sydney, house just a few weeks before she turned 18 because she felt she had nowhere else to go.
"I felt obligated. I didn't feel like I had a choice," JC told the court.
JC denied being happy Lynette was gone.
"I didn't want the responsibility of cooking, cleaning, looking after Lyn's children who she loved. I wondered where she was.
"I wanted her to come back so I could go and live my life as a 17-year-old."
Questioned by defence barrister Pauline David, she rejected that her claims she had been Dawson's sex slave were a good way to sell an unpublished book she had written about her life.
She also denied the book titled The Schoolgirl, Her Teacher and His Wife reinvented her life with Dawson to destroy him.
Dawson and JC married in 1984 and separated in 1990.
The former teacher and rugby league player has pleaded not guilty in the judge-alone trial to murdering Lynette.
The crown case is that he killed her and disposed of her body because of his affair with JC.
Ms David claimed Dawson might have failed his wife as a husband, but he did not kill her.
She said the police investigation into Lynette Dawson's disappearance had been flawed and they had failed to follow up alleged phone calls from her and sightings of her after she had supposedly been murdered.
JC is set to return to the witness box on Monday.
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