The incident happened between 12.45pm and 1pm on Sunday 11 August 2013 in the area of Barnes Close, the Railway Gardens and near to the Factory Shop car park.
The victim, a 40-year-old woman from Sturminster Newton, was involved in an altercation with a younger woman and sustained minor injuries to her legs and arms.
Detective Constable Elaine Leader, of West Dorset CID, said: “There were several people in the area at the time and I am appealing to them to come forward to help us ascertain exactly what happened.
“I ask witnesses and anyone with information about the incident to contact Dorset Police as soon as possible.”
A 13-year-old local girl has been arrested and released on police bail pending further enquiries.
Witnesses and anyone with information should call Dorset Police in confidence on 101 quoting incident number 11:232. Alternatively, call the free and anonymous Crimestoppers line on 0800 555 111 where mobile phone tariffs may apply.