A SHOP owner proved a teen prom queen's relatives "stole the wrong dress from the wrong person" by catching the thieves after a two-month-long sting.

Chantilly Weddings' owner Shona Berry staked out formals in Toowoomba until she finally spotted her $880, one-off lace-and-crushed-taffetta number �?despite attempts by the thief to "re-birth" her designer outfit with a homemade, flashy pink addition.

"It meant a lot to us that dress because there was a lot of work that went into the lace and the beading," Mrs Berry said.

"And I felt very strongly about someone stealing from my shop."

She recalled a young woman trying on the dress, the most expensive item she carried.

"I had in my mind right from the start I was going to find this dress at formal time and embarrass this particular girl, whoever she was," she said. "I wanted to ruin her formal as she ruined my day. I was just put out that someone stole a dress from me."

She and her seamstress set out to recover the frock, methodically working through several school formals about two months after the theft occurred.

It turned up at the Centenary Heights State High School end-of-year bash �?slightly altered but otherwise unmistakable.

"It was black and white originally but (they) had put pink over the exclusive lace panel," Mrs Berry said.

"You know your own dress. We had our own trademark on it, there was no trouble identifying it at all."

Police were called and officers escorted the humiliated teenager in the stolen dress from the formal for questioning.

This week the girl's stepmother, Deanna Turnbull, and her older sister, Tania May Hounslow, appeared in the Toowoomba Magistrate's Court on charges relating to the theft.

Both pleaded guilty and Hounslow was sentenced to five months' jail. Turnbull was ordered to serve 50 hours' community service and pay $225, half the cost of restitution. The teen who wore the dress, Sommer Rose Turnbull, 17, was not charged with any offence.

Detective Sergeant Rebecca Bradley of Toowoomba CIB said it was, without doubt, the most unusual case she had worked on.

"The lady from the shop was really on the ball.

"She brought in a picture of what the dress looked like and decided to go to the formals to find it," Det Sgt Bradley said.

Mrs Berry said she did not know how the dress was stolen.

"I think they had a decoy to occupy me in the dressing room while they made off with the dress from the showroom. I think it went out in a bag," she said.