They died in a two-vehicle crash on State Highway 1.
The major route is now closed between Ward and Kekerengū.
Two of the injured people were airlifted to Wellington Hospital.
They will be tested on emergency procedures if a train derails.
The crash happened on Friday evening.
Police said they did not believe there was an ongoing risk to the public.
It's understood the spill is at the ferry terminal.
Watch it here.
The vehicle rolled 200 metres down a hill in Marlborough.
