Police are seeking a masked man who approached a woman in Christchurch this morning and pointed what is believed to be a firearm at her head.

The man approached her from behind about 6am on Riccarton Ave, near the Cricket Oval and pavilions, and pressed something to her head.

The woman screamed and pulled away, and the man ran off.

He was last seen running towards Deans Ave.

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The offender is described as slim with dark hair, under 160cm tall, and aged in his 30s or 40s.

He was wearing a baggy grey hoodie, dark trousers and a beige Halloween-type mask, and was carrying what appeared to be a small dark-coloured firearm.

"At this stage, this is being treated as an isolated incident and officers are focused on catching the man responsible," senior sergeant Vaughn Lapslie said.

Police would like to hear from anyone who may have been in the area at the time as well as anyone who recognises the offender's description, or who has any information about this incident.

Please contact Christchurch Police on 03 363 7400, quoting reference number P029029690.

If you prefer to remain anonymous, you can leave information with Crimestoppers on 0800 555 111.