As torrential rain and strong winds battered Sydney's south, Paul Smith handed his colleague his wallet and keys and said, "Don't do what I'm about to do".

The 43-year-old, who was working as a supervisor at the Lucas Heights Waste Management and Recycling Centre, stepped into a stormwater drain that had become blocked in an attempt to clear sticks, mulch and other debris.

Paul Smith and his daughter Emma.

Mr Smith's colleague, Daniel Malone, told an inquest that the water level was at Mr Smith's waist as he entered the drain at about 2.40pm on March 24, 2014.

Mr Malone had not seen the drains before and as Mr Smith handed over his things, he said, "Don't do what I'm about to do".