An Australian woman who was wrongly detained in the United States for three weeks is back home in Adelaide.

Texan authorities arrested 21-year-old Laura McKenzie over a visa mix-up last month.

She says she spent time in various detention centres and was forced to sleep on a concrete floor and wear dirty underwear and clothes.

Ms McKenzie says she had no idea how long she would be held until a US judge dismissed her case because mistakes had been made.

"They kept telling me it was just a couple more days and I'd be out and everyone said 'Oh you'll be out really quickly and yet I was still being kept day after day," she said.

"It's so scary because you would think there's all these other people that have been in there for months and it's like 'What if that happens to me?'."