"A person was seen running across the road on the motorway," the police spokeswoman said.
"The person was hit by a car and was reported to have serious injuries."
St John said two ambulance crews were sent to the incident at 11.55pm.
"We treated one patient in a serious condition and transported them to Auckland City Hospital," a St John spokeswoman said.
Police also received a few calls about a single mattress blocking the Northern Motorway near Northcote Pt at 11.30pm.
"We also received a call advising that a vehicle was driving along the motorway dragging a mattress behind," the spokeswoman said.
"It appears that it was stuck in the wheel and the vehicle had to pull over into a bus bay to remove it."
She said police arranged for contractors to collect the mattress.
One patient was treated for minor injuries at the scene, St John said.