A man covered head-to-toe in dark clothing and armed with a knife carried out a frightening aggravated robbery of a Post Shop in Christchurch yesterday.

Christchurch police are appealing to the public for information which might help catch the offender.

The man entered the Kiwibank Post Shop on Queenspark Drive, Parklands about 1pm before presenting a knife and demanding cash.

"The man, described as about 160cm tall, left with an undisclosed sum of cash and was last seen running towards Chadbury St," police said in a statement.

"He was wearing a black hoodie with "CCC" on the front and a black peaked cap with a dark-coloured bandana covering his face."

Anyone who saw the alleged offender in the area during this time, or who might have any other information which might help police with their investigation is urged to contact them.

Members of the public can call 105 and ask for Detective Constable Alfeld at Christchurch Police, or call Crimestoppers anonymously on 0800 555 111.