Picton's port was evacuated and ferries delayed while emergency services responded to a volatile gas leak from a rail wagon tank in the nearby railyard.
KiwiRail reported the leak at 6.10am and initially said the leak was on a truck on the Interislander ferry Kaiarahi.
It later said the leak was at a railway yard near the ferry terminal and the area had been evacuated.
Fire and Emergency has confirmed the leak of highly flammable LPG from a 25,000-litre tank.
Two fire crews from Picton in breathing apparatus, sprayed water on the tank to contain the leak and minimise the risks of explosion.
Two more crews from Blenheim were called in, as well as the Nelson hazardous materials unit. Police were also assisting.
KiwiRail chief operating officer Todd Moyle said the tanker was venting through a valve, as it was designed to do when there was some sort of fault.
"We've implemented our standard procedures of isolating the site and putting an 800m cordon in place ... while we're those getting technicians to site to fix the fault."
There was a very limited risk of explosion, he told Morning Report.
The cordon was lifted mid-morning.
The ferry Kaiarahi, which had been due to arrive in Picton at 7am, was able to dock at about 9.40am.
Port Marlborough chief executive Rhys Welbourn said he was relieved things were back to normal and the situation was is controlled.
SH1 between London Quay and Dublin Street has reopened.