Mahendraparvata was discovered using laser-scanning technology by the University of Sydney’s archaeology research centre in the country.
A team from Australian newspaper the Sydney Morning Herald broke the news of the previously-unknown city, which joins together a network of previously-known buildings and new discoveries.
The archaeologists told the SMH that they believe some of the temples of Mahendraparvata, on the slopes of the Phnom Kulen mountain, have never been looted, and could contain countless treasures.
Archaeologists and mapping experts have uncovered more than 20 previously unrecorded temples, as well as evidence of ancient canals and roads.
Mahendraparvata is thought to have existed around 350 years before Angkor Wat.
Damian Evans, director of the University of Sydney’s centre in Cambodia said: “We see from the imagery that the landscape was completely devoid of vegetation.
“One theory we are looking at is that the severe environmental impact of deforestation and the dependence on water management led to the demise of the civilisation... perhaps it became too successful to the point of becoming unmanageable.”