Mr Forbes escaped in his car and reported the incident to police.
Hawke's Bay District Health Board's chief operating officer Warrick Frater said the hospital was "extremely upset" by the incident and had apologised to Mr Forbes.
The patient was not a mental health inpatient. Mr Frater said the man was in a ward with a serious medical condition that made him unable to control his behaviour, but staff were unaware he could be violent. His condition had since deteriorated and he was now in intensive care.
The hospital would be using CCTV footage to investigate how the patient came to be outside. Police were also investigating.
Later that afternoon, near Holt Place in Waipukurau, a Central Hawke's Bay Mail deliverer in his early teens was shot in the arm by a 10-year-old boy with a homemade bow and arrow.
The delivery boy had walked past, felt a sting in his left shoulder and then saw a stick on the ground.
CHB Mail manager Bruce Doran said the boy did not require medical attention.
"He probably got a bit of a fright, but if it hit him in the face, or the eye, it would've been a different kettle of fish."
Mr Doran said the incident was reported to police and Youth Aid. "It wasn't a serious [bow and arrow], it was a kid's one. But it's more the principle ."