The port at Picton, including the ferry terminals and the surrounding industrial area, has been evacuated due to a gas leak.
KiwiRail has confirmed an LPG gas leak was detected on a train wagon in a yard at the port soon after 6am.
Fire and Emergency says two crews from Picton arrived first on scene, while crews from Blenheim and a Hazmat truck from Nelson have also been called in.
It is unclear whether there is any ongoing danger to the public.
Police have confirmed cordons are in place around the port, and several nearby streets.
A KiwiRail spokesman is warning Cook Strait ferry passengers to expect delays to services as the incident is resolved.