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 Mr 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.
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image of Thomas Fessy
BBC West Africa correspondent
Thousands attempt to cross this stretch of Sahara desert to reach the coasts of North Africa, and from there Europe. I remember the exhaustion of migrants whom I met in Agadez in 2011.
Most were in their 20s and determined to continue their journey – despite the war in Libya at the time. “In our country, our talent is useless,” one young Nigerian said. “But in Europe, we’ll be able to thrive on it,” he added.
Agadez is still several hundred kilometres from either Libya or Algeria. There is nothing in between but sand – a perilous desert where the extreme heat can kill. Traffickers extort large sums of money from migrants seeking a one-way ticket for a better life. They all know that death may be on the way, but they’re ready to risk it, clinging onto hope.
Security officials have said the second vehicle broke down as it was on its way back to Arlit to get spare parts.
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 10km (six miles) from the Algerian border.
Speaking from Arlit, a centre for uranium mining north of Agadez, Mr Alhacen said he had experienced the worst day of his life when he found the bodies.
“The corpses were decomposed; it was horrible,” he said.
“We found them in different locations in a 20km (12mile) radius and in small groups, often under trees, or under the sun. Sometimes a mother and children, but some lone children too,” AFP news agency quoted him as saying.
They were given Muslim burials where they were found, he added.
Given that at least 48 of those found were children or teenagers, Mr Alhacen said it was possible they were on their way to low paid jobs in neighbouring Algeria.
It is not clear which countries the migrants came from.
About 80,000 migrants cross the Sahara desert through Niger, according to John Ging, director of the UN Office for the Coordination of Humanitarian Affairs.
“They are basically economic migrants. They are in search of work. They are so impoverished that they have to make these hazardous journeys,” he told the BBC’s Newsday programme.
Niger is one of the world’s poorest countries and frequently suffers from drought and food crises.