The 81-year-old man was outside his St Ives home talking to neighbours when he heard his smoke alarm going off and went inside to investigate.

Inspector John Paul of Fire & Rescue NSW said the man paid a high price for his decision.

"It's alleged the gentleman was outside with neighbours. Upon hearing his smoke alarm he's re-entered the building and unfortunately doing so has cost him his life," he said.

"People should be reminded they should never re-enter a home once its alight."

Neighbours called emergency services who arrived at the Blackburn St property where they found the home "engulfed in smoke and flames", according to police. 

It took an hour for the fire to be extinguished. 

When firefighters searched the property, they found the body of the man.

The cause of the blaze is under investigation and a report into the incident will be prepared for the coroner.