The senior NSW Police sniper involved in the Sydney Lindt Café siege has told 60 Minutes he believes the death of hostage Tori Johnson was a preventable tragedy.

In December 2014, lone gunman Man Haron Monis held 18 innocent people hostage inside Sydney's Lindt Café for over 16 hours.

Tragically, two of the hostages – café manager Tori Johnson and Sydney barrister Katrina Dawson – were killed in the terrorist incident.

In an exclusive 60 Minutes interview, Mark Davidson has told reporter Liz Hayes that missed opportunities and a lack of information prevented him from shooting Monis when he had the opportunity.

"We could have saved the hostages, we could have saved Tori at least," an emotional Davidson said.

"In my opinion, I believe Tori was a preventable death."

Davidson says hours earlier in the siege he had Monis in his gun sights for over ten minutes and could have killed the terrorist.

"I could see Monis' head, you could see the shiny sort of bald scalp and the Islamic black bandana with white writing across the front that went across his forehead," Davidson said.

Davidson was positioned in the Westpac Bank diagonally across from the Lindt Café.  He says with the help of another sniper he had a plan to first breach the glass window of the bank.

The initial blast would have provided a hole for Mark to shoot through, allowing him to fire a precision shot almost simultaneously into the Lindt café, killing Monis and avoiding further tragedy.

Mark Davidson didn't pull the trigger though because he was concerned Monis may have also had a bomb which could have detonated with the sniper's shot.

But 60 Minutes revealed that five hostages who had earlier escaped the café had all told police they didn't see wires coming out of the gunman's backpack, an indication the terrorist didn't have a bomb.

This was intelligence that would have been critical to the sniper's plan of action.

"Certainly it was a major factor in everyone's mind and the detonation of something like that in a space like that is catastrophic," he said.

For Mark Davidson, information about the potential bomb could have made the difference between taking and not taking the shot.

But the information never reached Mark Davidson or his sniper colleagues.

"There was an achievable option to shoot and kill him and save the hostages at that time," Davidson told Hayes.

"It's upsetting because that's what we're there for."

"It's the call I made at the time, and I have to live with it," he said.

But Mark also had to consider that if he did take a shot at the man he believed was the terrorist and got it wrong, he could be charged with murder.

 "You're weighing up shooting someone and if you think you're not justified then you're facing murder charges yourself."

After more than 16 hours, the Sydney Lindt café siege came to a dramatic and devastating end when Monis ordered Tori Johnson to his knees.

For Mark Davidson, it is a searing memory and an image he will never forget.

It started when Monis fired his weapon at no-one.

"I definitely saw Tori flinch and his body was engulfed by orange flash of light which I believed was the muzzle flash of the shotgun," Davidson told Hayes.

 "[Tori] sits back up, doesn't turn around, doesn't attempt to get eyes on Monis. It was like he was resigned to his fate. That his soul has resigned itself to leaving his body at that time."

Seconds later Monis fired the shot which killed Tori Johnson. Immediately afterwards, police stormed the café and Monis was gunned down in a shower of bullets.

Tragically, Sydney barrister and mother Katrina Dawson was also killed in the incident.

Believing he could have saved the lives of hostages has caused Mark Davidson enormous grief.

The weight of that horrific day led him to give up his career as a sniper and he was medically discharged from the New South Wales Police Force.

"There [are] things I would have liked to have done differently, but that's the past. I can't change it," he said.

"I can just hope that we, the collective, have learned important lessons from it and we minimize the chance of it happening again."

To watch 'The Sniper' in full and for more from 60 Minutes, visit the official website.