Granddaughter held over Adelaide murder

A 19-year-old Queensland woman has been arrested over the "shocking" murder of her elderly grandfather in his Adelaide home earlier this month.

The teenager and her 21-year-old female friend, both from Redbank Plains, are scheduled to appear in the Brisbane Magistrate's Court on Monday where South Australian police will apply for their extradition.

It's expected the pair will then be extradited to Adelaide where they'll be formally charged with the murder of 81-year-old Robert Whitwell.

He was found dead in his Craigmore home on August 8.

South Australia Police say the scene inside the house was "very unusual".

"The circumstances surrounding the murder are quite shocking," Detective Inspector Greg Hutchins told reporters on Saturday.

"It was one of those scenes that was very difficult to read."

Det Insp Hutchins said it took a team of forensic investigator and crime scene pathologists to work out what had occurred inside the house.

Investigators initially believed the 81-year-old had likely died of natural causes but there were a number of factors which couldn't be explained.

After it was determined Mr Whitwell had been murdered, officers flew to Queensland and conducted interviews and searched properties on Friday, Det Insp Hutchins said.

The granddaughter and her friend were subsequently arrested. No further arrests are anticipated.

Det Insp Hutchins on Saturday said Mr Whitwell's family was "devastated".