A short time later he dragged her off her bike, across a fence and into an area of bush, Ms Neill said.
A witness from a nearby property went to help her and the assailant ran away. A police dog team was unable to find him but police said they were following "positive lines of enquiry".
Investigations into the incident today included completion of a scene examination at Sir Basil Arthur Park.
Police also examined a Mazda hatchback vehicle located there.
The victim was interviewed again today and was coping well, Ms Neill said.
"Police and the girl's family would like to acknowledge the actions of the witness, whose intervention may well have prevented a much more serious outcome."
Timaru police were continuing to appeal for information from anyone who was in the vicinity of Sir Basil Arthur Park between 7.45pm and 8.20pm yesterday, or who had any information regarding the incident. They were asked to contact Timaru Police on (03) 687 9808.