Two fire investigators are currently in attendance at the scene to ascertain the fire.
A St John spokeswomen said one male patient in his 20s has been transported to Auckland Hospital in a critical condition.
Police have set up cordons in repose to the fire, which has left the road partially blocked off.
Departing school students from the Kelston area were asked to gather at these cordons.
Students from Kelston Girls High School and Kelston School for Deaf are asked to make their way to the cordons to meet parents or caregivers, while students from St Leonards Primary School and Kelston Boys High School are asked to use an alternative route home.