Police impartiality is crucial into the investigation into the double shooting in Invercargill, says a former top cop.

Constable Benjamin Peter McLean has been charged with the murder of Verity McLean, with whom he had three children.

He has also been charged with the attempted murder of Garry Duggan, who is in a stable condition in Dunedin Hospital.

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Former police investigator Lance Burdett told Newstalk ZB's Larry Williams that Invercargill Police are using an independent investigator.

"That's somebody at a senior level, probably around detective inspector, overseen by the detective superintendent, just to make sure everything's done correctly."

Burdett said it will be no different to any other internal investigation, and will help eliminate any emotional connections.

He also suspects McLean might be treated harsher than the average alleged offender, as often police aren't softer on their own.

"You're dealing with perhaps another investigator or another cop who knows all the tricks and traps."

Senior officers from outside the Southland district, Inspector Steve Wood from Dunedin and Detective Superintendent Peter Read from Canterbury, have been called in to oversee the homicide investigation.

Southern District Commander Superintendent Paul Basham said the move was important to ensure an impartial investigation.