A Northern Territory mine has been fined almost half a million dollars over the "tragic and foreseeable" death of a mine supervisor after he was engulfed by a 48,000 cubic meter section of an unstable pit wall in 2019.

The Bootu Creek Mine's operator, OM Manganese, pleaded guilty in September to the single charge of failing to comply with its workplace safety duty, which in turn exposed workers to the risk of death or serious injury.

Craig Butler, the mine's supervisor at the time, was trapped under hundreds of tonnes of debris while overseeing workers mining manganese ore in an open cut pit, 130 kilometres north of Tennant Creek.

Workers that morning had observed sediment slipping and rocks falling at the southern end of the wall, prompting the mine supervisor to carry out an assessment.

"Tragically he died at the scene," Judge Thomasin Opie said on Wednesday during sentencing at the Darwin Local Court.

"The debris extended across the pit to a width of 80 metres.

"Two other workers were located in dangerously close proximity but were not physically harmed."

Mine operator ignored warnings

In handing down her sentence Judge Opie said the mine operator had ignored a risk assessment matrix and a number of reports that made it clear the pit was unstable, and that in the days leading up to the incident, a number of workers had observed the wall "starting to lose it's structure".

"A machine operator had observed some rocks falling from the pit slope about halfway up the wall and heard it being recorded to the shift's supervisor," Judge Opie said.

"Based on the geotechnical information and reports of visible pit slopes instability available on the 24th of August 2019, OM's risk matrix would have classified the risk as extreme and unacceptable, requiring the work to stop."

Judge Opie took into consideration that the mine company had chosen an "aggressive design" for the wall, which meant it was "steeper and carried a high risk of failure".

She said that two years previously a geo-technician had made a number of recommendations to safeguard the wall design, but the mine ignored that advice.

She also said a ground control management plan – which included advice on pit wall monitoring – was not implemented.

Company accepts 'full responsibility'

In making her final decision, Judge Opie said she'd taken a number of victim impact statements – detailing the profound impacts on Mr Butler's friends, family and colleges — into consideration.

She said two of Mr Butler's colleagues who were working in the pit at the time detailed the "shock and devastation" of seeing the wall collapsing "knowing that Mr Butler would not survive".