The search for missing four-year-old August “Gus” Lamont will resume on Tuesday, with fresh boots on the ground from the Australian Defence Force and South Australia Police, following a devastating blow to investigators earlier last week.

Gus vanished from his family’s remote sheep station, about 40km south of Yunta in South Australia’s Mid North, around 5pm on Saturday, September 27.

He was last seen playing on a mound of dirt near the homestead. When his grandmother called him in just 30 minutes later, he was gone according to the family.

What followed was one of the largest search operations in South Australian history — police, SES volunteers, and the Australian Army scoured more than 60,000 hectares of harsh terrain by air and ground.

The search was called off last week, but on Monday police confirmed they would resume searching for Gus at Oak Park Station, 43kms south of Yunta, on Tuesday.

“The search will concentrate on an expanded area outside of the zone already searched extensively,” a statement read.

“There continues to be regular and close engagement with the Lamont family who are continuing to assist with the investigation.”

A second footprint once thought to be a vital clue in the case was ruled out last week, leaving authorities with little to no evidence and no clear direction.

The print, discovered near a dam and subjected to intense scrutiny using infrared drone technology, was confirmed on October 7 to be unrelated to Gus.

The only clue that remains is a single foot print that was found on September 30, 500m from where Gus was last seen

Former SES volunteer Jason O’Connell, who walked the property alongside Gus’s father, said the case defies logic.

“My heart breaks for (the father),” O’Connell told 7NEWS.

“It’s been searched. (Gus) is not there.”

O’Connell was given police approval to use his tracking skills to help with the search.

He told 7NEWS that he has never seen a case like this before, where there has been not been a single trace of the person who has gone missing.

“I just don’t get how (Gus) vanished like that,” O’Connell said.

Assistant Commissioner Ian Parrott last week acknowledged the heartbreaking decision to scale back the search, citing expert advice that survival was unlikely.

“While we’ve all been hoping for a miracle, that miracle has not eventuated,” he said.

“We’re confident that we’ve done absolutely everything we can.”

Gus is described as shy but adventurous, a good walker who had never left the property alone before.

He is Caucasian with long blond curly hair and was last seen wearing a grey sun hat, cobalt blue long-sleeve Minions shirt, light grey pants, and boots.

Anyone with information is urged to contact police.