The father of a mentally-ill man who killed three neighbours in a shooting spree unlawfully failed to keep the keys to his gun safe away from his son, police have alleged.

But Guiseppe Corbo's lawyer said his client complied with firearms laws by keeping his guns in a locked safe and his ammunition in a separate locked safe.

"Keys are not mentioned in the legislation," the lawyer, Gilbert Aitkin, told Adelaide magistrates court on Friday.

Corbo, 71, has pleaded not guilty to three firearms offences including failing to keep a firearm secured.

He is the father of Donato Corbo who shot dead three members of a family in Hectorville in Adelaide in April 2011.

He also wounded two other people, including a police officer investigating the incident, but was found to be not guilty of all charges by reason of mental incompetence.

Prosecutor Andrew Nicholls said police were not saying the gun safe was not locked or that the ammunition case was not locked.

Rather, the prosecution alleged the keys were in a place known by the son and therefore his father did not "do enough" to secure the items.

"The keys were in a place where (the son) knew where they were and he was able to access there, the rest is history."

Mr Aitkin said the shootings were committed by a mentally-ill person who did not know the difference between right and wrong.

But police were "intent on this case running to trial" claiming that the son's actions "somehow sheets home" to his client, who had complied with firearms legislation.

He noted another magistrate had thrown out manslaughter charges against his client, a charge based virtually on the same facts as the present charges.

The trial is continuing.