A New Zealander who joined the infamous Waco cult in Texas before a siege that cost more than 70 lives has spoken about her four months as a prisoner at the compound.

Photo: AFP

Grace Adams was 29 when she left for Waco in 1990 with her younger sister Rebecca, who died in the fire that engulfed the centre in 1993.

She told CBS News all women and girls older than 12 were told to have sex with cult leader David Koresh in order to go to heaven.

But when she knocked on his door at 3am one morning, he flew into a rage, claiming 'God made all his sexual selections'.

"It was then that he decided that I needed to be isolated in a cabin," she said. "I was treated like a prisoner.

"My mind did get messed up. He slapped me around ... on my face."

She said she was put her under 24-hour guard in solitary confinement in a 10x8-foot room, where she was fed from a bowl on the floor.

She said she begged Koresh to let her leave but he refused - until he learned her visa had expired and that immigration authorities may come looking for her.

Ms Adams said he told her: "God said you can go now. It's time for you to go."

CBS said she spent more than two weeks at a California hospital in a psychiatric ward before flying home to New Zealand.

But her sister stayed behind at the compound and Ms Adams watched the siege on TV.

"It was awful," she said. "It's awful seeing those people burned alive."