Geophysical testing at an Adelaide factory identified as the potential resting place of the Beaumont children has uncovered a "small anomaly" suggesting a hole was dug on the site, police have revealed.

Key points: A "small anomaly" suggests a hole was dug at the factory once owned by person of interest, Harry Phipps

A "small anomaly" suggests a hole was dug at the factory once owned by person of interest, Harry Phipps Two brothers say they dug the hole for Mr Phipps the same year the Beaumonts disappeared

Two brothers say they dug the hole for Mr Phipps the same year the Beaumonts disappeared Police will search the site within weeks

The three Beaumont children — Jane, 9, Arnna, 7, and Grant, 4 — went missing from Glenelg beach on January 26, 1966.

Adelaide businessman Harry Phipps, who died in 2004, once owned the Castalloy site on Mooringe Avenue at North Plympton that is currently at the centre of police attention.

Police first investigated Mr Phipps as a person of interest in 2007.

Detective Superintendent Des Bray said that investigation prompted two brothers to come forward and tell police they had dug a hole for Mr Phipps at the site the same year the Beaumonts disappeared.

He said after earlier searches, police had begun a "discrete investigation" last year to minimise harm to the family, in parallel with a Channel 7 probe.

Superintendent Bray said police then "worked collaboratively" with the media outlet and Flinders University, who have analysed data gathered from what is called ERT (electrical resistivity tomography) testing at the Castalloy site earlier this month.

"That analysis identified a small anomaly in the middle of the block consistent with where the brothers said they had dug, but there was no obvious reason for that anomaly," he said.

"There could be an innocent explanation or it could well be the hole they dug.

"They are not suggesting it was a grave, but certainly it was sufficient to merit further investigation and that's what we've committed to do."

Hole about 50 metres from previous search spot

Police conducted a radar search of the North Plympton factory site in 2013. ( SA Police )

Superintendent Bray said the brothers were only boys at the time they dug the hole.

He said the new spot was about 50 metres away from where police have previously focused.

"We've searched in the areas they've nominated before but for a number of reasons they now believe it's further along. We actually think they're right," Superintendent Bray said.

"It does appear to have been a hole which is about six square metres, and between about 2.5 and three metres in depth."

An excavation of the site is expected to be carried out within weeks, but police have spoken of the need to "temper expectations".

"There's never been anything to prove that the Beaumont children are in the hole," Superintendent Bray said.

A sketch of the man witnesses said they saw with the Beaumont children the day they disappeared. ( Supplied: SA Police )

The update on the case came after police revealed on Monday that a new search of the site would be carried out.

At the time the children went missing, several witnesses told police they had seen them with a tall, tanned, thin-faced man with short blond hair.

Police said Mr Phipps was still considered as a person of interest, along with about a dozen others.

"It's probably had more people nominated as a potential offender than any other case that I'm aware of," Superintendent Bray said.

A $1 million reward remains on offer by the State Government and police for information about the children's disappearance.

Anyone who has any information relating to the Beaumont case is urged to come forward.