The fatal car crash happened at Molong in June 2023.
(ABC News)
Corrective Services NSW released a statement saying the man was found unresponsive in his cell at the Metropolitan Remand and Reception Centre in Silverwater on Tuesday.
A spokesperson said he was "immediately given first aid and taken to hospital but was pronounced deceased".
"As a matter of protocol, Corrective Services NSW and NSW Police investigate all deaths in custody regardless of the circumstances," the spokesperson said.
Mr Armstrong was due to make his first court appearance in July.