Heavy rain and slips made the road through the Awakino Gorge impassable in places.
On-call police officer Constable Donna Potroz received a call about 1.40am about a major slip blocking the gorge.
Multiple vehicles, including a patient transfer ambulance with the unwell baby on board, were stuck by the slips, police said.
St John said it was carrying the patient by ambulance from New Plymouth to Waikato Hospital, because weather conditions prevented an air ambulance from being dispatched.
National operations manager Chris Haines said a team of neonatal care specialists from Health New Zealand was on board, providing essential care to the unwell baby.
He said the support of Potroz, who decided to travel alongside the ambulance officer and medical team, was important - in that she provided access to vital communications.
"Along the way, we encountered some road obstructions which meant we had to evaluate our options and adjust our plans, with the support of police."
When conditions improved, an air ambulance airlifted the baby to Waikato Hospital.
"We are incredibly grateful to everyone who played a part in the care of our young patient," Haines said.
Taranaki acting area commander Inspector Darin Haenga said that Potroz put the safety of others ahead of her own. "When situations like these present themselves people rely on our support.
"Constable Potroz was brilliant, helping all these people trapped by the slips to feel safe and be safe.
"I'd also like to acknowledge how well served our communities were by roading crews at the scene, and the ambulance and Health New Zealand staff, who worked hard through the night."
