The Vietnamese and Cambodian governments have joined together to celebrate the inauguration of a border marker between the two countries.

The official marking of the border comes as the two countries commemorate 45 years of diplomatic relations.

Cambodia's Prime Minister Hun Sen said at the ceremony he wanted the border to become "a mechanism of good friendship forever", while Vietnam's Prime Minister Nguyen Tan Dung

reportedly said the event marked a joint success of the two nations in building a borderline of peace, friendship and co-operation for development.

While the official ceremony was accompanied by a joint celebration at the new border, some Cambodian groups are opposed to the move.

Lowy Institute visiting fellow Dr Milton Osborne says acknowledgement of the border with Vietnam has traditionally been a sore point between the Cambodian government and opposition.

"The issue goes back even before the period of French colonialism, where Cambodia has over the centuries consistently lost land both to Vietnam and to the Thais," Dr Osborne told Radio Australia's Connect Asia program.

"Deeply ingrained within many Cambodians is a view that Vietnam is continuing to occupy territory which once was Cambodian."

The low-lying Mekong Delta region in southern Vietnam is often referred to as "Lower Cambodia" by Cambodian nationalists.

Dr Osborne believes the border marking signifies an important step towards both Cambodian and Vietnamese friendship and future demarcation of marine borders.

Cambodia and Vietnam fought a series of full scale battles in the late 1970s after the Vietnam War ended.

Vietnamese forces occupied Cambodia (formerly Kampuchea) after the fall of the Khmer Rouge.