Rescue worker Almoustapha Alhacen said the corpses were in a severe state of decomposition and had been partly eaten, probably by jackals.
Those found are thought to be migrant workers and their families. Most were women and children.
Niger lies on a major migrant route between sub-Saharan Africa and Europe.
But among those who make it across the desert, many end up working in North African countries.
According to Almoustapha Alhacen, one of the vehicles that the migrants were travelling in broke down some time after they left Arlit at the end of September or beginning of October.
It appears that some of the group set out on foot, including up to 10 people who made it back to Arlit and raised the alarm, he said.
It was reported on Monday that five bodies had been found.
On Wednesday, volunteers and soldiers working in searing heat found other corpses about 6 miles from the Algerian border.
Speaking from Arlit, a centre for uranium mining north of Agadez, Almoustapha Alhacen said he had experienced the worst day of his life when he found the bodies.
They were given Muslim burials where they were found, he said.
Given that at least 48 of those found were children or teenagers, Almoustapha Alhacen said it was possible they were on their way to low-paid jobs in neighboring Algeria.
It is not clear which countries the migrants came from.
“There were no clues. My guess is that the children were madrassa [Islamic school] children, being taken to Algeria to work. That is the only explanation that I and others can find for such a large number of children having travelled together,” Almoustapha Alhacen said.
About 80,000 migrants cross the Sahara desert through Niger, according to John Ging, director of the UN Office for the Co-ordination of Humanitarian Affairs.
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