Opera House staff show bass trombone thieves how to exit building

The Sydney Opera House has been accused of lax security after thieves stole a $9000 musical instrument following a New Year's Eve concert.

Police have released photos of a man who may be able to "assist with inquiries" over the theft, which happened shortly before 3am on January 1.

Stolen was a bass trombone belonging to freelance musician Colin Burrows, who had performed at a gala concert staged by Opera Australia, Fairfax Media reports.

"Somebody has walked all the way through the main warm-up room under the Concert Hall stage to a locker room to take it," he said on Facebook.

Mr Burrows, who was attending a private staff party in the iconic building's Green Room when the theft occurred, said security footage showed an Opera House staff member showing the thieves how to exit the building — although there is no suggestion the staff member was involved in the theft.

"After everything that has happened in Sydney recently, how two people can still sneak into a building like this is quite disturbing," he said. "The Opera House has since told me they hold no responsibility for this."

Mr Burrows said he will have to fly himself to the United States to buy another trombone, which will cost about $US7000 (AUD$8950).

An Opera House spokeswoman declined to comment.