Robert Penny, 83, of Malvern, had been due to face the Melbourne Magistrates Court for a committal mention after being charged with murdering his wife, Margaret, 58, and hairdresser Claire Acocks, 49, in May 1991.

But defence lawyer Emily Clark said a medical certificate had been sent to the court last week asking Mr Penny be excused from appearing.

Mr Penny suffers from diabetes, emphysema, cirrhosis of the liver and vascular disease.

Magistrate Charlie Rozencwajg agreed to adjourn the case to July 20 when a committal hearing date was expected to be set down.

Mr Rozencwajg said the exact number of witnesses to be called during the expected three-week hearing had yet to be determined.

The magistrate extended Mr Penny's bail, which had been granted to him during a Supreme Court hearing in April.

The prosecution case against Mr Penny is that he murdered both women by stabbing them with a sharp implement at the Old London Coiffure Hairdressers, on the corner of Julia and Bentinck streets, on May 3, 1991, before turning off the lights and locking the front door.

It is alleged a tail comb - which has a long, thin metal handle - was one of the weapons used, along with a Bowie knife, but no weapon has ever been found.

Homicide detective senior constable Thomas Hogan told the earlier bail hearing that telephone intercepts and listening devices in place in February 2014 had picked up a phone call between Mr Penny and his granddaughter, Afra, where a hitman theory was discussed.

Detective Hogan said Mr Penny told Afra: "Would a hitman pick up a comb and kill two women?"

The prosecution case also relied on phone calls within three to fours hours of the murders between Mr Penny and his son, Tony, who was in the UK at the time, and his daughter, Jacqueline, who was in Melbourne.

Mr Penny allegedly told his children about the weapons he believed had been used in the murders - information allegedly only the killer would know.

Another crucial conversation allegedly took place at about 6.50am the morning after the killings when Mr Penny went to see Mrs Acocks' husband Peter looking dishevelled and told him Claire had died very quickly.

Mr Penny allegedly told Mr Acocks that his wife had been killed with a tail comb.

Detective Hogan said crime scene examiners did not arrive at the hairdressing salon until between 30 minutes to an hour after Mr Penny's alleged visit to Mr Acocks' home. 

The detective agreed there was no forensic evidence linking Mr Penny to the murders, there had been a number of suspects over the years, including some who had died, and the prosecution case was entirely circumstantial.

Mr Penny has denied any involvement in the murder and a partial DNA sample found on an item at the crime scene did not match his DNA.