China has cracked down on opposition in Hong Kong after the city was rocked by huge and often violent democracy protests in 2019.

Beijing has imposed a national security law that criminalised much dissent and new "patriots only" political rules that vet the loyalty of those standing for office.

The first election under these rules was held on Sunday for the city's legislature, with a historic low turnout recorded.

"We, the Foreign Ministers of Australia, Canada, New Zealand, and the United Kingdom, and the United States Secretary of State, noting the outcome of the Legislative Council elections in Hong Kong, express our grave concern over the erosion of democratic elements of the Special Administrative Region's electoral system," the nations said in a joint statement.

Hong Kong's new rules reduced the number of directly elected seats and tightened control of who could stand for office.

"These changes eliminated any meaningful political opposition," the allies said.

"We also remain gravely concerned at the wider chilling effect of the National Security Law and the growing restrictions on freedom of speech and freedom of assembly, which are being felt across civil society."