The seriously injured man, in his 50s, was found at a block of flats in the Darwin central business district on Friday morning.
"It was quite a confronting scene for police to attend," Detective acting Senior Sergeant Michael Budge told reporters.
The man was taken to the Royal Darwin Hospital in a critical condition and died on Wednesday about midday.
"This is now a murder investigation," Det Snr Sgt Budge said.
"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.
"We have not yet made an arrest."
A crime scene was established at the Tomaris Court flats on Friday and detectives collected CCTV from the area.
"We know some people went to the service station and reported an incident," Det Snr Sgt Budge said.
Those people were the man's flatmates. They continue to assist police and are also persons of interest in the investigation.
Det Snr Sgt Budge said it was likely a domestic incident and the man had two children.
"This is a very sad time for them," he said of the man's family.
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