More than 20 people have been investigated in relation to a sexual assault in the Ross Creek Reserve, but a year on from the attack the offender remains unidentified and police have listed the file inactive.

About 9.15pm on December 1 last year, a 23-year-old woman ran to a Cannington Rd, Dunedin, house and banged on the door for help after being tied up and sexually assaulted in the reserve in what police described at the time as a random ''blitz-type attack''.

Detective Senior Sergeant Kallum Croudis said yesterday officers were no longer actively investigating the matter.

Police files can be marked inactivate when leads are not available or suspects cannot be located.

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The files are not closed, but resources are moved on to other matters.

Det Snr Sgt Croudis said there was no further information police could provide and the public was informed of everything over which they could help the police investigation.

Police released an identikit image of a man they wanted to speak to in relation to the assault.

However, police found the man was not involved in the matter.

''Assessments of at least 20 persons nominated were made,'' Det Snr Sgt Croudis said.

''Some required more investigative work than others to eliminate from them from this inquiry.''

The Identikit image also bore a resemblance to another released in relation to a sexual assault which took place days later near Lincoln University, in Canterbury.

Police found no evidence to suggest any connection, Det Snr Sgt Croudis said.

Police would not comment directly on whether the public should be concerned about the offender attacking again.

''The public exercising or using any bush reserve should use vigilance, common sense and judgement,'' he said.

At the time, Det Snr Sgt Croudis said such attacks were rare in Dunedin and he could recall five or six during the past two decades.

The assailant was described by the victim as a tall and skinny European wearing black shoes, a black T-shirt, a pair of black ''Skins'' compression wear and a black balaclava.

He tied the victim using black cable ties and fled the scene after being distracted during the attack by a light-coloured dog.

• Dunedin police (03) 471-4800, Crimestoppers 0800 555-111.