A man who was allegedly assaulted last week in the Darwin CBD by unknown assailants has died in hospital, NT Police said, prompting a murder investigation with the main suspect or suspects still outstanding.

Police said last week that a man had been seriously injured overnight Thursday or Friday morning and they were alerted to the incident when a woman entered a retail business stating that her partner had been assaulted.

The man was located at a nearby residential complex and taken to hospital in a critical condition with serious head injuries. That complex was not identified by police but is understood to be the Tomaris Court flats on Smith Street.

It was unclear who exactly is alleged to have carried out the assault.

Police established a crime scene at the block of flats last Friday and were reviewing CCTV and speaking with residents.

“Investigations are progressing and we have spoken with residents of the complex who were able to assist us in establishing a timeline of what happened,” said Detective Acting Senior Sergeant Michael Budge.

“We have not yet made an arrest. This is now a murder investigation.”

Anyone with information is asked to contact police on 131 444, quoting reference number 9974017.