"An unavoidable head-on collision occurred," Coroner Michael Robb said.
"Mr Ikinofo-Edwards was found to have THC, the active ingredient of cannabis in his blood, together with methamphetamine.
"Those drugs, in combination with fatigue, were the likely cause of him driving on the wrong side of the road."
Ikinofo-Edwards was convicted and sentenced for the bodily injury caused to Rendell and Milla Klopper, and for causing the death of Te Mana.
Charges for which pleaded guilty.
NZ Transport Agency (NZTA) recommends any child under the weight of 9kg, or less than a year old, should be in a rear-facing child car seat, Coroner Robb said.
"The reason for this recommendation is that a child's skeleton and muscles cannot adequately support their head and spinal cord until they are 12 months or older."
Coroner Robb said Te Mana's neck was fractured as he was thrown forward in the sudden impact of the head-on collision.
They were fatal injuries and the baby died at the scene.
A rear-facing child seat reduces the impact of such a collision on a child's head and neck by providing a support and a barrier to the forward momentum, Coroner Robb said.
Coroner Robb made two recommendations after Te Mana's death.
Firstly, that there be continued education and enforcement in relation to the dangers of driving while distracted and/or fatigued and while under the influence of drugs.
Secondly, that there be continued education on the NZTA recommendation that infants under 9kg in weight or younger than 12months of age be secured in rear-facing child seats.
