Disturbing final diary entries of couple abandoned in shark-infested waters after boat trip forgot them

It has been 28 years to the day since Tom and Eileen Lonergan first went missing in Australia.

The American couple from Louisiana were both in their early 30s when they disappeared in Australian waters on 25 January 1998.

Both avid travellers, the pair loved exploring new scuba-diving sites around the world, which led them to the Great Barrier Reef, one of the world's most popular diving locations.

Tom, a US Air Force Reserve pilot, and Eileen, a school teacher, joined 26 others on a diving trip to St. Crispin Reef through the dive company Outer Edge Dive; however, the two were accidentally left behind.

It took two long days after the group went diving for them to realise that Tom and Eileen were missing, leading to a large-scale search operation by the navy, police and volunteers.

Just 16 days before they disappeared, however, Eileen reportedly wrote a concerning passage in her diary.

Eileen reportedly wrote a concerning passage in her diary before they went missing (QLD Police)

She penned: "Tom hopes to die a quick and fairly painless death and he hopes it happens soon. Tom's not suicidal, but he's got a death wish that could lead him to what he desires, and I could get caught in that."

Six months prior, it was reported that Tom also wrote in his diary that he felt 'as though my life is complete and I'm ready to die'.

"As far as I can tell, from here, my life can only get worse. It has peaked, and it's all downhill from here until my funeral," it said.

However, Eileen's family has previously dismissed the speculation of alleged suicidal intention and said the words have been taken out of context.

Her father, John Hains, believed that the couple had died as a result of dehydration, drowning, or a shark attack.

Search efforts reportedly found a dive slate with a plea for help written on it, saying:"[Mo]nday Jan 26; 1998 08am.

"To anyone [who] can help us: We have been abandoned on A[gin]court Reef by MV Outer Edge 25 Jan 98 3pm. Please help us [come] to rescue us before we die. Help!!!"

The couple's bodies were never recovered (Tom and Eileen Lonergan)

The message was believed to have been written the day after they were left behind and the day before anyone realised they were missing.

The boat's captain, Geoffrey 'Jack' Nairn, was later charged with manslaughter, though he was later acquitted after being found not guilty. He said he had given a crew member the task of performing a headcount, which resulted in a miscount when two passengers jumped into the water halfway through.

The company pleaded guilty to negligence and went out of business.

Although their bodies were never found, some of their equipment had washed up on shore in the months that followed their disappearance.

Eileen's wetsuit was found about a month later, with tears in it, thought to have been caused by coral. Other items recovered include dive jackets, tanks and one of Eileen's fins.

Theories about their fate include drowning or dehydration, being eaten by sharks, a suicide pact, or faking their deaths, though these two theories were dismissed by a coroner.