An episode of "The Simpsons" in which the cartoon American family visit Tiananmen Square is missing from the Disney+ streaming service in Hong Kong, adding to concerns about mainland China-style censorship in the city.

It comes at a time when authorities are clamping down on dissent, with curbs on speech becoming a norm in the international business hub and ensnaring global media and tech giants.

Disney+ has made rapid advances since it was launched 18 months ago, reaching more than 116 million worldwide subscribers.

The Hong Kong version started streaming earlier this month and eagle-eyed customers soon noticed the conspicuous absence of "The Simpsons" episode 12 of season 16.

First airing in 2005, the episode features the family's trip to China in which matriarch Marge Simpson's sister tries to adopt a baby.

In one scene, the Simpsons are at Beijing's Tiananmen Square, the site of a deadly 1989 crackdown against democracy protesters. The cartoon shows a sign there that reads "On this site, in 1989, nothing happened" -- a satirical nod to China's campaign to purge memories of what happened.

It then shows Marge's sister standing before a tank, referencing the famous photo from the Tiananmen crackdown of a lone man standing in front of a tank.

The episode also contains pointed comments about Tibet -- where Beijing has been accused of religious oppression -- and the Cultural Revolution, a devastating period of upheaval in the last decade of Mao Zedong's rule.

It is not clear whether Disney+ removed the episode, was ordered to by authorities or if it was offered in Hong Kong to begin with.

The entertainment giant has not responded to requests for comment, nor has Hong Kong's government.

When AFP checked Disney+'s Hong Kong channel on Monday episodes 11 and 13 of season 16 were available but not 12.

Disney+ started streaming in Hong Kong in November and and eagle-eyed customers noticed an episode of 'The Simpsons' featuring China was conspicuously absent Disney+ started streaming in Hong Kong in November and and eagle-eyed customers noticed an episode of 'The Simpsons' featuring China was conspicuously absent Photo: AFP / Peter PARKS

Until recently, semi-autonomous Hong Kong boasted significant artistic and political freedoms compared with mainland China. But authorities are currently transforming the city in the wake of huge and often violent democracy protests two years ago.

Among the slew of measures are new censorship laws introduced this summer that forbid any broadcasts that might breach a broad national security law that China imposed on the city last year.

Censors have since ordered directors to make cuts and refused permission for some films to be shown to the public.

Those rules do not currently cover streaming services but authorities have warned that online platforms fall under other rules, including the new national security law.

Last week, Hong Kong's Beijing-appointed leader Carrie Lam vowed to "proactively plug loopholes" in the city's internet and introduce "fake news" regulations.

Her comments added to concerns that China's "Great Firewall" -- a sprawling internet and news censorship regime -- could be extended to Hong Kong.

Content that satirises China is still available on other streaming platforms in Hong Kong.

Netflix's Hong Kong channel is still showing "Band in China", an episode of the cartoon series "South Park".

In that episode, one of the characters ends up in a Chinese labour camp and much of the show lampoons the willingness of American brands to adhere to Chinese censorship rules to make money.