It remained unclear how many people died in the disaster in PNG's Enga region, but the national government said more than 2000 people may have been buried alive.
A UN estimate put the death toll at about 670.
A 14-day window provided by local authorities for search and rescue will end on Thursday, the International Organisation for Migration (IOM) said in a statement.
"The provincial government will cease searching for bodies due to public health risks and the potential for new landslides, as the soil remains unstable.
"The unrecovered bodies will be declared missing persons, and the landslide site will be designated a mass burial site with monuments erected."
Geological experts from New Zealand have recommended that thousands of residents be evacuated from a larger area due to the risk of another landslide, the IOM said.
"These areas are highly volatile, with cracks forming in the ground, necessitating immediate evacuation."
Treacherous terrain and tribal unrest in the area meant heavy equipment and aid were slow to arrive, and PNG government officials a week ago ruled out finding survivors under the rubble.
The disaster site will be quarantined with access restricted to prevent the spread of disease from decaying bodies, according to the IOM.
PNG Prime Minister James Marape last week blamed extraordinary rainfall and changes to weather patterns for multiple disasters in the Pacific Island nation this year, including the landslide.
