The second baby was born 38 minutes after the first one at 6.32pm with its umbilical cord around its neck and with no heartbeat.
A post-mortem found the baby died of intra partum asphyxia or a lack of oxygen.
But an expert adviser, specialist obstetrician and gynaecologist Dr John Short, who reviewed the case for the HDC said he had "serious concerns" about the monitoring of the baby's heart rate and the lack of action between the birth of the two babies "when his heart rate deteriorated considerably and the trace becomes grossly abnormal".
Short said an instrumental delivery should have been carried out immediately after its sibling was born. He said the baby's heart rate was "so frankly non-reassuring" that urgent delivery was indicated.
He also criticised the minimal notes taken by the doctor on the day which was also "very poor practice".
In his finding, Hill was very critical of the obstetrician and said it was an extremely sad case, and he expressed his deepest sympathies to the parents.
Hill recommended the doctor make a written apology to the family for her breach of the code and that the Medical Council of New Zealand consider reviewing the doctor's competence if she ever wanted to return to practice.
The DHB's care was also found to be "inadequate" and it has since implemented a number of recommendations which included reviewing its guidelines for delivering twins, training staff and purchasing a second newborn Resuscitaire unit.