Australian citizen Kevin Yam, a lawyer and longtime Hong Kong pro-democracy activist, was targeted in letters that carried his photo and alleged national security offences.

The letters, first reported in The Guardian newspaper, offered a reward of HK$1 million (US$128,000) to anyone who could provide information about him and the allegations or "take him to Hong Kong or Australia Metropolitan Police".

They were sent to homes next to two Melbourne locations cited in the notices as being linked to Yam, the paper said.

"The Australian government will not tolerate surveillance, harassment or intimidation against individuals or family members here in Australia -- this undermines our national sovereignty and the security and safety of Australians," said a spokesperson for Foreign Minister Penny Wong.

"We are raising our concerns directly with Chinese and Hong Kong authorities."

In a message on social media, Yam said he would continue to live his "everyday life".

"I will not voluntarily return to Hong Kong before it is free," he said.

"I will not kill myself."

Beijing expressed "strong dissatisfaction and resolute opposition" to what it called "Australia's blatant interference" in Hong Kong's law.

"Hong Kong affairs are purely a Chinese internal affair and brook no interference from any outside powers," foreign ministry spokeswoman Mao Ning told a regular briefing, when asked about the case concerning Yam.

"China urges Australia to earnestly respect China's sovereignty... and create favourable conditions and atmosphere for the sustainable development of China-Australia relations," she added.

- 'Days in fear' -

The letters were not signed but asked for information to be sent to a Hong Kong police email address used for tip-offs on wanted people.

The Hong Kong government said it does not issue anonymous letters.

Recipients should "remain cautious to verify the authenticity" of any such letters and seek help from local police if needed, a government spokesman told AFP.

Hong Kong law enforcement will pursue overseas suspects in accordance with the law and "take every measure" to stop them from continuing to endanger national security, the spokesman added.

Yam reportedly returned to Australia in 2022 after two decades in Hong Kong.

In 2023, Hong Kong chief executive John Lee called on eight overseas activists including Yam to turn themselves in for violating the national security law.

At the time, he backed a police decision to offer HK$1 million for information leading to their arrests, and warned the activists to surrender or "spend their days in fear".

All eight fled Hong Kong after Beijing imposed a sweeping national security law on the financial hub in 2020 to quell dissent after huge, sometimes violent, pro-democracy protests in 2019 were quashed.