TOKYO (AFP) - A Japanese firefighter moonlighting as a gaming YouTuber had cold water poured on his career by public officials who uncovered the secret side hustle by identifying his voice in the lucrative videos.

An anonymous tip-off prompted Wakayama city in western Japan to launch an investigation into the 33-year-old's channel, which had around 15,000 subscribers.

But because the firefighter never appeared on screen, a local official had to painstakingly comb through the clips for vocal clues to his identity, the city said.

The man's salary was cut in punishment for contravening a law limiting public workers' engagement in commercial activities.

Over a 10-month period, he had uploaded more than 300 videos, typically showing him playing multi-player strategy games - racking up around 1.15 million yen (S$13,465) in advertising revenue.

"We don't necessarily think it's a bad thing that he was a YouTuber," city official Hidetaka Amano told Agence France-Presse on Wednesday (Jan 12).

"But it's the fact he was profiting from ads, some of which could be inappropriate in nature."

His actions had "betrayed the trust of residents in Wakayama", the city official added.