Mary-Kate and Ashley Olsen are being sued by their former interns for ''wage theft''.

A lawsuit - filed in Manhattan Supreme Court - has been brought against the 29-year-old fashion designers' company Dualstar by interns, who claim they weren't paid for their work, the New York Post reports.

The lead plaintiff, former design intern Shahista Lalani, said in court documents that although the Olsens were nice when they came into the office of fashion label The Row, she was overworked by their head technical designer during her time there in 2012.

She said: ''They [Mary-Kate and Ashley] are really nice people. They were never mean to anyone. They're business people.

''[But the head technical designer] was very demanding. I was doing the work of three interns. I was talking to her all day, all night. E-mails at nighttime for the next day, like 10 pm at night.''

The suit is seeking class action status and estimates up to 40 past and present interns could join.

Lalani's responsibilities during her 50-hour weeks included: ''inputting data into spreadsheets, making tech sheets, running personal errands for paid employees, organising materials, photocopying, sewing, pattern cutting, among other related duties,'' according to court papers.

Lalani added in the filed documents: ''When we weren't doing something, they'd be like, 'Organise the buttons in the back by colour code.' You don't get a set 15-minute break.

''You're like an employee, except you're not getting paid. They're kind of mean to you. Other interns have cried. I'd see a lot of kids crying doing coffee runs, photocopying stuff.''

A spokeswoman for the Olsens' company told The New York Post that Dualstar was not aware of the suit.