The law, granting the immigration chief powers to bar people from boarding planes to and from the city, came into effect on Sunday.

"Under this amendment, individuals, including foreigners, might be unable to leave the territory," Canada's travel advisory Twitter account said.

The tweet linked to an updated advisory Ottawa published on Tuesday that specifically mentioned the new rules.

"You may not be aware that you are the subject of movement restrictions until you try to leave Hong Kong," the advisory stated.

"In the absence of clarifying legislation, these new powers may relate to investigations into an individual, their family or an employer, and criminal and civil matters."

So-called "exit bans" are often used by mainland China against activists who challenge authorities.

They have also trapped business people involved in commercial disputes.

Some western nations, including the United States and Britain, have criticised the law but Canada is the first to explicitly mention the exit-ban risk in a travel advisory.

Hong Kong's government says the law will not be applied to people leaving the city and is aimed at stopping illegal immigrants arriving in the city.

But the wording does not limit the power to arriving flights or immigrants, and legal experts say it could also be applied to anyone leaving Hong Kong.

In a potential recognition of those concerns, Hong Kong's government in April said it would draft subsidiary legislation specifying that the law would only be applied to inbound flights.