A man has been seriously injured in a balcony fall after refusing to open the door to police and trying leave, authorities say.

The 32-year-old is in a stable condition in Sydney’s Royal North Shore Hospital on Tuesday night, having tumbled from the third floor of a unit.

Officers arrived at a complex on Cooper St in Waterloo about 1.30pm to speak with the man as part of “ongoing police investigations”, but say they were refused entry.

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“Whilst police remained outside the unit, the man attempted to exit the unit via the balcony and fell,” NSW Police said.

Police found the man and supported him until paramedics arrived.

NSW Police could not provide any more information into the circumstances of the incident when contacted by 7NEWS.com.au, with those details to form part of the critical incident investigation that is now underway.

A critical incident is something involving police that results in death or serious injury.

7NEWS is not suggesting any wrongdoing by police, only that the investigation is underway.

“The investigation will be subject of an independent review by the Professional Standards Command and oversight by the Law Enforcement Conduct Commission,” police said.