The 43-year-old man died at the scene of the Haymarket building, despite neighbours, police officers and paramedics applying CPR for more than 30 minutes.

Police were called to the Castlereagh St building around 3.15am on Easter Sunday after neighbours found the man unconscious in the hallway of the 13th floor.

The man was unresponsive by the time police got there. Neighbours were performing CPR.

Residents told police they'd heard a "melee" or a "wrestle" taking place outside their doors in the hallway, Detective A/Superintendent Paul Dunstan said on Sunday morning.

Police and emergency services pitched in with CPR for approximately 35 to 40 minutes but the man died at the scene.

A 20-year-old man was arrested at the scene. He's now assisting police with their inquiries and is expected to be charged on Sunday afternoon with offences "akin to murder", Supt Dunstan said.

Supt Dunstan said the men returned to the apartment together around 2am.

Police don't know what happened inside but ultimately a fight spilled out into the hallway.

"It's made enough noise to arouse the neighbours, who have contacted police," Supt Dunstan said.

The incident started with punches then escalated into a wrestle, but no weapons were used, he said.

Detectives are investigating the death, including by obtaining CCTV.

It was a busy night for Sydney's police.

Elsewhere in inner Sydney, a man was stabbed just after midnight in a brawl at Pyrmont, police say.

The 18-year-old had stab wounds to his buttocks after a brawl allegedly involving two large groups.

Most of those involved dispersed before officers arrived. Police are investigating.

Around an hour later, two men were robbed at gunpoint in the early hours of Sunday morning.

The men, aged 32 and 35, had their watches, jewellery and credit cards taken by a masked man at Glebe around 1.15am.

Police have not been able to locate the man.

Nearby, around 1.20am, an officer was bitten on the arm and grabbed in the groin as he tried to arrest a man in Chippendale.

The officer was patrolling Regent Street where three men were reported to be acting suspiciously.

One of the men ran when he was found to have bank cheques and a credit card that didn't belong to him.

The sergeant caught up to him but was allegedly assaulted as he tried to restrain him. He's since been discharged from St Vincent's Hospital.

The 25-year-old man has been charged with assaulting a police officer and other offences and will appear before Parramatta Local Court on Sunday.

Australian Associated Press