On Tuesday he began throwing up "foul" faecal smelling vomit in a shower before collapsing and dying.
His cause of death was listed as cardiac arrest caused by a "massive" gastrointestinal bleed.
Health and Disability Commissioner Anthony Hill said the care provided by Northland DHB showed a "moderately severe" breach of required standards.
The doctor in charge of the man's care, known as Dr A, persisted with the diagnosis of gastrointestinal bleeding despite indications his condition was not typical.
"Dr A missed several opportunities to question and review his diagnosis and to consider the appropriateness of further clinical investigations. In my opinion, Dr A did not provide services to Mr B of an appropriate standard."
Northland DHB was guilty of not having clear guidelines or clinical criteria for gastroscopy referral, Mr Hill said.
"The care provided by the DHB was suboptimal, highlighted systems issues, and was a moderately severe departure from expected standards."
The DHB was given a series of recommendations, including orders to improve its gastroscopy referral system and train staff in keeping an open mind when a patient's symptoms do not fit their diagnosis.