An elderly man has died after spending a cold night trapped in a trench outside a home in Sydney's west.

The man's son discovered his father outside the Glenmore Park home early on Friday morning.

It is believed the man, 91, fell into the trench at some point during the evening when the temperature dropped to just 7 degrees.

The son contacted emergency services who found the man trapped in the trench and submerged in water.

The man could not be revived and he died at the scene.