Lawyer Bernard Collaery says Australia had direct knowledge of the dangers facing the murdered Balibo Five journalists, citing a damning report suggesting Australian intelligence knew Indonesia viewed reporters as a “hurdle to be got over” in keeping its invasion of East Timor secret.

Collaery has published his long-awaited book Oil Under Troubled Water, a forensic account of Australia’s relationship with its tiny developing neighbour, Timor-Leste. The lawyer describes the relationship as a “tortuous” and “grim” tale of exploitation, missed opportunity, culpability and complicity.

The book’s release comes amid Australian government threats and the criminal prosecution of Collaery for his role in exposing the bugging of Timor-Leste’s government offices during sensitive oil and gas negotiations.

Collaery’s book treads carefully around the criminal proceedings he faces for legal reasons. But it raises renewed questions about what Australia’s foreign intelligence service knew of the dangers to five journalists murdered by Indonesian forces in Balibo in the lead-up to the invasion of then Portuguese Timor in 1975.

Collaery quotes a secret assessment from the then British ambassador to Indonesia John Ford that points to high-level and immediate intelligence sharing between Australia and Indonesia’s intelligence service, then known as Bakin, in 1975.

Ford’s report described information obtained by “the Australians” showing they had knowledge of “clandestine activity” by Indonesian troops in East Timor in the lead-up to the invasion. It said Indonesia’s principal fear was that their activities in Timor would be exposed.

“A particular hurdle to be got over is a plane load of Australian journalists and politicians who are due to visit Timor, apparently at Fretilin request, to investigate allegations of Indonesian intervention,” Ford said in his 1975 report. “The information from the Australians is sensitive and should not be played back to them or repeated to other missions.”

The Australian Secret Intelligence Service has fought to keep records on Indonesia’s occupation secret, though the report quoted in Collaery’s book has previously been unearthed by the British researcher Hugh Dowson.

Collaery said he had found Ford’s report almost by accident while searching through the UK National Archives.

“While writing the book in Cambridge, I spent a great deal of time at the royal archives,” Collaery told the Guardian.

“Much of the unrevealed Australian record was there, and it was challenging to read. But when I found ambassador Ford’s letter, merely by chance, I got up and went out to the canteen and had a cup of tea, I felt so sick. I felt so sick. I’d assisted [Balibo widow] Shirley Shackleton with [Balibo victim] Greg [Shackleton]’s estate and I knew the families, and I couldn’t believe it.”

Clinton Fernandes, a leading expert on Timor-Leste who has fought Asis to release Balibo records, said the Ford report was “striking” because it hinted Australia’s foreign spy agency “knew more than has previously been admitted”.

“Indonesian operatives in the 1970s viewed massacres and assassinations as normal modes of behaviour,” Fernandes told the Guardian. “If the journalists were ‘a hurdle to be overcome’ then their fates were sealed even before they landed in Timor, and Asis knew it.”

In his book, Collaery said there “seemed little doubt” the journalists’ lives could have been saved if Australia had directly warned Indonesian leaders.

“It is for any inquiry to determine whether those dealing with Bakin knew the news team was headed to Balibo and shared that knowledge,” he said.

He also expressed surprise that Australia, given its knowledge, did not warn journalists of the threat they faced in Timor.

“Given the history of Indonesia’s ruthless suppression of media dissent at home and in West Papua, it is surprising that Australia and Britain did not give more direct warnings to the media contingents that continued to arrive at Dili in September and early October 1975 in a blaze of publicity about alleged clandestine Indonesian military activity,” Collaery wrote.

“More damning, given the top-level intelligence liaison between Australia and Indonesia revealed by Ambassador Ford, is the apparent failure to warn [Indonesian intelligence chief] Lt General Yoga Sugama of the anticipated fallout were state-sponsored harm to befall foreign journalists.

“Clearly the intelligence relationship, at least between Australia and Bakin, was robust enough for such a démarche.”