An inspector from the National Offshore Petroleum Safety and Environmental Management Authority is "satisfied on reasonable grounds that an activity may occur at the premises that, if it occurred, would involve an immediate and significant threat to the environment".
The structural integrity of the 10 oil tanks used to store petroleum are under question after one was found by Jadestone to have a 3.5cm hole in it.
After Jadestone noted an oil slick during a transfer between tanks it sent a remote operated vehicle for an inspection and found the hole. It reported the spill to the regulator, as required under Australian law.
"Based on this known failure it is now reasonable to conclude that the structural integrity of the remaining cargo oil tanks is uncertain," the inspector wrote.
The Environmental Prohibition Notice is more serious than the directions previously issued to the company in March and November of 2021.
It believes there could be a "significant" release into the sea. Jadestone must plug the hole in the tank and also ensure structural integrity of the other tanks.
When the company told the London Stock Exchange's AIM Monday morning UK time shares fell 20% in same day trade. It said it expected production guidance to be within the lower range.
When Thailand's PTTEP operated the field a 2009 oil spill, totalling some 2000 barrels of oil per day, was the largest environmental disaster of its kind in Australian waters. Oil reached as far as Indonesia and the spill became subject to a vast class action lawsuit by Indonesian seaweed farmers.
Jadestone already suffered two strikes from NOPSEMA last year.
NOPSEMA issued Jadestone Energy an Occupational Health and Safety Improvement Notice in September after a dropped object led to a worker rolling their ankle and requiring a medevac from its Montara platform in the Timor Sea the previous month.
It was one of two accidents aboard Montara that month, with the first seeing a worker burned from a boiler; however, as the worker was not medevaced, Jadestone said it was under no obligation to inform the regulator.
In March it was hit with a direction over corrosion concerns, and NOPSEMA said then it remained unhappy with the level of work undertaken by the company to improve the situation on the ageing oil platform.