Heater manufacturer Seacombe Pty Ltd and its production manager Marcel Rappold, who has no gas qualifications, were hit with the fine in the Dandenong Magistrates' Court.

The court heard Mr Rappold was not a licensed or qualified gasfitter but did service and maintenance work on a gas log fire after the homeowner complained it was noisy.

Almost four months later a licensed plumber and gasfitter checked and tested the heater and identified a carbon monoxide spillage while the appliance was in use.

An investigation by Energy Safe Victoria found high levels of carbon monoxide were present when the appliance was operating.

The investigation also found Mr Rappold had fitted a part that was not approved for use in the gas log fire.

Energy Safe chief executive officer Leanne Hughson warned carbon monoxide poisoning could cause serious injuries or death before people even knew it was present.

"Carbon monoxide is a silent killer that can't be seen or smelt when it builds up in enclosed spaces, so it's incredibly fortunate the homeowner used the gas heater sparingly or the situation could have ended in tragedy," Ms Hughson said.

"Qualified gasfitters are trained to test for the presence of carbon monoxide - that helps to keeps people safe while unlicensed work can put people in danger."

Magistrate Steven Raleigh also highlighted the potentially deadly consequences of gas heaters leaking carbon monoxide.

In convicting and fining Mr Rappold $5000 and Seacombe $4000, the magistrate noted the need to make it clear to unregistered and unlicensed people not to work with gas.