Police say impartiality is crucial following a double shooting in Invercargill, allegedly by one of their own.

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

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

SEE ALSO: Invercargill devastated after double shooting - Mayor

Former Police investigator Lance Burdett told Larry Williams Invercargill Police have hired 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 Mr McLean might be treated harsher than the average 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."

LISTEN ABOVE AS LANCE BURDETT SPEAKS WITH LARRY WILLIAMS