Up to 750km of coastline was put at risk of contamination from possible oil spill by BP’s plan to drill in the Great Australian Bight, newly released documents show.

Government documents released under freedom of information laws show a major oil spill in the sensitive seascape would pollute up to 750km of beaches and shoreline, according to BP’s own modelling. The company also thought drilling could disrupt migration of the endangered southern right whale.

Two letters from the National Offshore Petroleum Safety and Environmental Management Authority (Nopsema) to BP in 2015 and 2016 show BP had failed to address the regulator’s concerns about managing these risks. The details suggest it would be costly for any oil company to drill the area in an environmentally responsible way.

BP withdrew its plans to drill in October 2016, citing better options for investment. But the company said the Bight remained a prospect and it still owns two of the four offshore leases that were subject to the original plans.

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The other two leases were passed from BP to Statoil last year and the Norwegian firm, which was a partner in BP’s initial project, intends to drill an exploratory well in one of them by October 2019. However, the Nopsema letters show any oil company seeking to drill that wild and remote corner of the Southern Ocean will face serious regulatory hurdles.

“Given the plan was never finalised, this material doesn’t represent the final views of BP or the regulator,” a spokesperson from BP said, adding that BP had no plans to reactivate its interest in the Bight. Chevron has also since dropped its own plans for the area.

In 2016, BP released modelling showing a spill could hit land as far away as New South Wales. The letters revealed that BP’s “worst case shoreline oiling scenario predicts oiling of 650km coastline ​at 125 days after the spill, increasing to 750km after 300 days”. Nopsema had raised concerns over BP’s ability to mobilise the people and equipment needed to clean up such a vast expanse of coast.

According to Nopsema’s letters, the crew on BP’s seismic survey saw migrating southern right whales and feeding sperm whales “on a number of occasions”. The oil company’s environment plan concluded whales may be exposed to underwater sound “at levels within the behavioural disturbance range”.

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BP had not evaluated the potential ecological consequences of the exposure. It concluded southern right whales (which are endangered) would simply change course to go around the area, but did not assess what that might mean for their ability to access food.

“This correspondence included a range of feedback that BP was working to address,” said the company spokesperson, when asked about these specific impacts.

Campaigners who fought BP’s proposal said the Nopsema letters showed particular difficulties associated with drilling in the Great Australian Bight which Statoil would struggle to overcome.

Jenny Webber, the campaign manager for the Bob Brown Foundation, said protecting the Bight’s wildlife from these risks would prove “prohibitively expensive”.

“That’s why BP and Chevron have withdrawn. Statoil must meet the same standards required of BP: its shareholders, not least the Norwegian government, need to know the company cannot reasonably meet the cost of keeping Australia’s southern ocean and coastlines safe from an oil spill which could take months to cap. Statoil should drop its plans,” she said.

Nathaniel Pelle, a senior campaigner at Greenpeace Australia, said: “Now we not only know how far it could reach, but how extensive an oiled coastline could be – 750km of oiled beaches is a horrifying thought.”

Pelle said this scale and the lack of a plan to recruit the necessary salvage crews “confirms the community’s concerns that an effective clean-up would not be possible”.

Even after BP was given a detailed rejection notice by Nopsema in November 2015, its resubmitted plan failed to comply, or had provided insufficient information, on 69 out of 88 required criteria, the documents show.

A Nopsema spokesperson said it was “common to see improvements in the environmental management approach proposed by titleholders for a particular activity as an environment plan assessment progresses”.

BP said the letters were part of an “ongoing process” and until the time it withdrew, it was “absolutely committed to working with the regulator to finalise our draft environment plan”.

Andrew Hopkins, an emeritus professor of sociology at the Australian National University who has consulted and written books on BP’s safety practices, said it would have cost BP “many millions” to comply with the regulator’s requirements.

BP said that compliance with the regulator had not made the project uncompetitive.

“Statoil must be able to see what will be required,” Hopkins said.

A spokesperson for Statoil said the company was developing an environmental plan to submit to Nopsema, which would be subject to consultation.

“We will only undertake drilling activity if we can do it safely. By the time we drill we will have spent more than two years planning this well to satisfy ourselves that we can operate safely and in accordance with Australia’s strict environmental and regulatory requirements. We have collaborated with CSIRO to study the local environment and this is taken into account in our plans. Our exploration project is also designed to protect the interests of fishing, aquaculture and tourism operators,” the spokesperson said in an emailed statement.

“As the world’s leading deepwater operator, we will draw on more than 45 years of experience successfully operating in similar offshore environments. We are speaking with local suppliers and stakeholders to ensure we properly understand the local conditions. We have decades of experience coexisting with other interests, including fisheries and tourism. We will continue to use a combination of local experts and global best practice throughout our exploration program. With careful planning and regulatory oversight, we are taking the time we need to prepare for safe operations.”

The freedom of information request to Nopsema was delayed by BP until well after its commercial interest in the region had cooled. The original document request was made in August 2016. The Bob Brown Foundation assisted CHN with the $386 fee for the request, but had no further input until asked to comment for this article.

• This is an edited version of an article first published on Climate Home News