Australia on Thursday offered protections for Hong Kongers living in the country in response to China's imposition of a tough national security law for the territory.

In steps certain to anger Beijing, Prime Minister Scott Morrison said his government was suspending its extradition agreement with Hong Kong and offering thousands of Hong Kongers in Australia on temporary visas a pathway to permanent residency.

He said the decisions were taken because the security law "constitutes a fundamental change of circumstances", adding that Beijing and Hong Kong authorities had been notified of the plans.

His announcement came a day after China opened a new office in Hong Kong for its intelligence agents to oversee implementation of the law imposed last week targeting acts of subversion, secession, terrorism and foreign collusion.

The law is the most radical change in Hong Kong's freedoms and autonomy since Britain handed the city back to China in 1997.

Morrison said about 10,000 Hong Kong citizens and residents in Australia on student or temporary work visas would be allowed to remain in the country for an additional five years regardless of the expiry date on their current visas.

"If you're a temporary visa holder, your visa will be extended to an additional five years from today, in addition to the time you've already been in Australia, with a pathway to permanent residency at the end of that period," he said.

"And we will also provide a five-year visa with a pathway to permanent residency for future Hong Kong applicants for temporary skilled visas, subject to meeting an updated skills list and appropriate marking."

Shortly before Morrison's announcement, Australia's foreign ministry warned its citizens in Hong Kong of the risk of detention under the new security law, which it called "vaguely defined".

It urged Australian nationals to "reconsider your need to remain in Hong Kong" if they have concerns about the new law.

Earlier this week the foreign ministry upgraded its travel warning for China, saying Australians faced possible "arbitrary detention" in the country.

China's embassy in Canberra blasted that warning as "completely ridiculous and disinformation".

In Thursday's notice, the ministry said the security law imposed on Hong Kong could be "interpreted broadly" and lead to Australians being transferred to mainland China for prosecution.

While noting that the full extent of the law and how it will be applied are "not yet clear", it said Australians "may be at increased risk of detention on vaguely defined national security grounds".

"You could break the law without intending to," the ministry added.