Police are searching for a man who had been spotted carrying a suitcase two kilometres from where a child's skeletal remains were found dumped alongside a Murray Mallee highway in South Australia.

The child is believed to have been aged between two and seven years old, and was originally dumped inside a dark grey suitcase on the side of the Karoonda Highway about two kilometres west of Wynarka.

Superintendent Bray said a man was seen by locals at Wynarka walking towards the highway between six to eight weeks ago carrying a suitcase between 7:30am and 8am.

"That person was about 60 years of age, average height, lightly built and neatly dressed and clean cut," he said.

"We have no description of the clothes he was wearing but he was carrying a dark-coloured suitcase in one hand ... he was not a local.

"There would not be many people walking around Wynarka carrying a suitcase at 7:30am in the morning."

Police said a motorist spotted the suitcase on the side of the road on Tuesday, pulled over to take a look, but did not report the grisly find until Wednesday.

Superintendent Des Bray said the remains themselves were found immediately behind a bush near the two-metre verge aside the Karoonda Highway bitumen.

"The child was originally in the suitcase and at some point we believe somebody has removed the child from the suitcase to there," he said.

A large contingent of police and SES members arrive at the Karoonda Highway near where a child's bones were found. ( ABC News: Alina Eacott )

"It's hard to say what happened, or why, but we know for certain that somebody has come and found the suitcase, opened it to have a look what's in there, tipped out the contents and at that stage become aware that it was most likely human remains."

He said up to seven people have since told police they had seen the suitcase on the side of the road where police believe it was first placed about two months ago.

One had dragged the suitcase from the bushes where police had originally believed it dumped and placed it in full sight on the verge.

Others had opened the suitcase, some had even "poked around ... not realising the importance of what was inside".

"We can say we believe the child died elsewhere ... placed in a suitcase and brought to this location," Superintendent Bray said.

About 20 State Emergency Service volunteers and a drone today combed the area in about a one-kilometre radius, after a search in a 50-metre radius of the site was conducted overnight.

Some articles of clothing were found, but because it is a highway police said it was hard to discern whether they were related.

Superintendent Bray said police were not prepared to comment on the other contents of the suitcase but "there was a variety of things inside".

The site where a child's skeletal remains were found alongside the Karoonda Highway in SA's Murray Mallee region. ( Supplied: SAPOL )

Police Commissioner Gary Burns said police would be undertaking investigations with people who regularly travelled the road or lived on properties in the vicinity.

He said the sex, and official cause of death had not yet been established.

Police are checking interstate missing persons databases because there is no-one on the South Australian database who is thought to match the discovery.

The bones were found about two kilometres west of Wynarka, about 130 kilometres east of Adelaide.

"Somebody must know somebody where a child has disappeared in suspicious circumstances," Superintendent Des Bray said.

"So we would encourage people to have a think about family, friends, acquaintances, people who live near them, perhaps somebody who has had concerns about children who live nearby and how they might be treated, and all of sudden a child's gone missing.

"It's impossible for someone not to know."