She said her client was addicted to methamphetamine and thought at the time he had been implanted with electronic fibres that monitored his movements as part of a Government anti-gang strategy.
He had a firearm with him to shoot down drones, Ms Maxwell Scott said.
Two West Auckland-based police officers were at Waitakere Hospital in Henderson dealing with an unrelated matter in October last year when the gunman pulled up in the ambulance bay at the emergency department.
Schuster got out of the car brandishing the single-barrel pump-action shotgun.
"The defendant has walked around the (empty) police patrol vehicle and fired three shots into it from different angles," court documents said.
"These shots have shattered the front passenger window, sprayed the vehicle exterior with dents and holes, and caused a large hole in the metal boot."
More than a dozen members of the public in a nearby waiting room were in direct line of any stray shotgun rounds, the summary of facts said.
Two constables left the hospital and saw what had happened.
One distracted Schuster while another retrieved a firearm from the car and he was arrested at gunpoint shortly afterwards.
He first came to police attention earlier that afternoon when he went to his brother's house in Glenfield and fired two shots inside.
After shooting at the television and wall, he threatened the occupants with the shotgun and bizarrely asked a female resident, who was in bed at the time, to Google "PHD and world population".
He thought they were all Government agents, Ms Maxwell Scott explained.
"One of the flatmates worked in fibre optics industry, so it all fell into place with his delusion," she said.
Schuster eventually left for the hospital in a car taken from the address.
His lawyer said the former butcher had begun using meth after the breakdown of a long-term relationship.
"As he became more addicted he became more paranoid and before he knew it he was out of control," Ms Maxwell Scott said.
"He's determined never to go anywhere near that drug again."
