Police in Auckland have issued an appeal for information after a man gained access to an inner-city apartment complex before sexually assaulting a woman, leaving her "incredibly shaken".

Detective Senior Sergeant Martin Friend from Auckland City CIB said the incident occurred on Sunday night at a residential complex on Albert St.

In a statement, he said the man gained entry to the complex and sexually assaulted a female resident in a communal area.

"Understandably the victim in this matter is incredibly shaken by what has happened and Police are ensuring there are support services engaged for her," Friend said.

Police are seeking information on this man, believed to be involved in a sexual assault.

"Our inquiries so far have located the offender boarding a bus, route 95B, at 6:50pm on Onewa Rd in Birkenhead and disembarking at Britomart at 7.08pm, on Sunday evening.

"The man has been in the central city and has eventually ended up at the residential complex on Albert St, where he has entered the building at around 9.20pm.

"Immediately after the incident has occurred, the man has left the area on foot."

CCTV stills released by police show the man on the bus and leaving the apartment, seemingly without his trousers.

Do you know this man?

Friend said an investigation was now under way and police were scouring other CCTV to track the man's movements.

"Our priority now is to identify and locate this man," he said.

"We believe the public will know who this man is and we need to hear from them."

The man on the 95B bus from the North Shore.

He assured residents in the central city that the matter was being taken extremely seriously and urged them to report any suspicious behaviour immediately.

"As part of this we remind people entering apartment buildings to practise good vigilance in ensuring entry doors close behind you and no one attempts to tailgate you into the building," Friend said.

Anyone with information is asked to contact Police via our 105 phone service or online at https://www.police.govt.nz/use-105, using Update My Report. Please reference file number 220724/6978.