Numerous people reportedly saw her being taken away, and one described Marais as "clearly distressed".
"It was really hard to watch as she was in a really bad way," the witness told the Daily Telegraph.
"They helped her down the stairs before putting her on the gurney in the ambulance. It was a hectic scene."
Marais won the Logie for Most Popular Actress in 2018. She also starred in the popular TV series The Wrong Girl and Love Child.
She was expected to appear in a rebooted Packed to the Rafters series this year, but abruptly quit the production in February.
"Jessica Marais has decided to step away from the new series Back to the Rafters for personal reasons," her management said.
"She thanks everyone involved on the show for their ongoing support and love, especially her 'Rafter's family', and sends them her very best wishes for the production."
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Marais also quit her role on Channel 9's Bad Mothers amid concerns for her health in 2018.
Marais has previously spoken about her battle with bipolar disorder.
"There's a history of bipolar in my family and I was diagnosed at one point with bipolar. I've had cognitive therapy training, so I choose not to be medicated," she told the Australian Women's Weekly in 2014.
Marais has a 7-year-old daughter, Scout.
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youth services: (06) 3555 906
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155
• CASPER Suicide Prevention
If it is an emergency and you feel like you or someone else is at risk, call 111.