A Melbourne man has been missing in Cambodia for more than a week after travelling to Phnom Penh with a colleague to meet Vietnamese human rights activists.

The man travelled to Cambodia as part of a planned meeting with activists. His disappearance has prompted concerns among people close to him, while authorities work to establish what happened after his arrival.

According to reports published on Tuesday, the Melbourne man and his colleague were in Phnom Penh when contact with him was lost. The circumstances surrounding his disappearance remain under investigation.

There are allegations that the man may have been abducted by Vietnamese authorities. Those allegations have not been established as fact, and investigators are working to determine the circumstances surrounding his disappearance.

The case has attracted attention because the man was reportedly travelling to meet Vietnamese human rights activists. His trip therefore involved contacts and activities that have raised questions about whether his disappearance could be connected to those meetings.

Cambodia is continuing to be the location at the centre of the investigation. Officials and people close to the missing man are seeking information about his movements and whereabouts since he arrived in Phnom Penh.

The Australian government has consular responsibilities when Australians become involved in serious incidents overseas. Consular officials can provide assistance to Australians and their families, although their ability to intervene directly in another country's legal or security matters is limited.

The disappearance also highlights the risks Australians can face when travelling overseas, particularly when travelling for meetings involving politically sensitive or human-rights-related activities.

At this stage, important details about the man's whereabouts remain unclear. Authorities have not publicly established exactly where he is being held, if he is being held by anyone, or what circumstances led to the loss of contact.

The allegations involving Vietnamese authorities remain allegations rather than confirmed findings. Further investigation and information from Cambodian, Vietnamese and Australian authorities will be important in determining what happened.

For the man's family and colleagues, the disappearance has created an urgent search for information. More than a week without confirmed contact has increased concerns about his welfare and whereabouts.

The investigation remains ongoing, and authorities are expected to continue gathering information about his travel arrangements, movements in Phnom Penh and contacts with people he was intending to meet.

The case is being closely followed in Australia because of the involvement of an Australian citizen and the international circumstances surrounding his disappearance.