Hamming spent the bulk of his 26-year police service at the Australian Bomb Data Centre, training agencies and businesses on how to respond to bomb threats.

An early interest in explosive devices would turn into an expertise for the Canberra-based policeman.

"I came from a rural area growing up, back in the day there was explosives being used for work on farms, I had friends on farms. There was fireworks that would go bang in the night, and being a typical country boy, I tended to probably do the wrong thing with those," he said.

The terror attacks in Bali occurred on October 12, 2002, and they left 202 people, including 88 Australians, dead.

Just after 11pm, a suicide bomber walked into Paddy's Bar and detonated an improvised explosive device (IED). Fifteen seconds later, a vehicle-borne IED exploded outside the Sari Club which was located 40 metres up the road.

"We had a sense of what we were walking into, there was so much media coverage. The TV stations, the news, were just bombing us with so much imagery of the scene, so we had some sense of what we were going to be seeing," Hamming said.

READ MORE

"But honestly, nothing can prepare you for when you actually got there."

The then-40 year old policeman arrived early on the Monday morning after the Saturday night attack, as part of the Australian Federal Police first response.

"On TV you don't get all of those other sensory inputs. It was the heat, and the humidity, but also the smells and all the other things, and you suddenly realised what a huge task was in front of us."

His role was to provide initial practical advice regarding how the device was triggered, how they were obtained, and what impact the explosive had on the environment. His team was able to reconstruct the bomb that was in the van outside the Sari Club.

Hamming established that the van contained 48 filing cabinet drawers filled with explosives and charges. His initial estimate was a bomb with a 500 kilogram effective load.

"We know now from the evidence recovered and submissions provided by offenders that there was well over a tonne of explosive material ... the way the bomb was constructed had some inherent flaws, whereby its construction caused the explosive not to fully function, so we didn't see the full explosive function," Hamming said.

Hamming's investigation made the stunning revelation, that if the full capacity of the bomb had been detonated, the death toll would have been significantly higher.

"It was a terrible event, but had it functioned to its full capacity, the death toll and the damage would have been so much bigger," he said.

"It's incomprehensible already to most reasonable people, how one could do that, but if the full 1.2 tonnes had detonated, the damage would have been vastly bigger."

Like many of the first responders, Hamming is still troubled by his memories of the devastation in the wake of the terrorist attack.

"It's one of those bizarre things in my mind, where I didn't personally suffer any loss, I wasn't on the scene, I wasn't subject to the blast. I was afterwards. And certainly from my professional perspective, it was a job to do," he said.

When thinking about those times that are now 20 years ago, a few moments stand out.

"When you see a scene, and you start to see things there, and you think: we know people died here, but who were they? What were they doing here? There was certainly one young couple who were killed at the scene that struck me particularly, because they were on their honeymoon.

"On reflection, I know that probably had a bigger effect on me than I would've like to have acknowledged."

First responders have high rates of post traumatic stress disorder, with an estimated 1.5 million Australians directly affected by PTSD.

"You'd provide that support for each other through dark humour, or 'how are you going mate?' It was very much like that: how are you going. You'd look after each other," he said of the first responder team.

The 20th anniversary of the Bali bombings has been complicated for families, victims and first responders, as one of the key bomb makers is set to be freed from prison in Indonesia after a reduction in his sentence.

Umar Patek has served just 12 years for his role in the terrorist attack, yet Indonesian prison officials say he has been deradicalised.

"It's not for me to comment, but I certainly empathise with the victims families who will be going through anguish because of this," Hamming said of Patek's potential release.

The Bali bombings investigation was a "watershed" moment for the Australian Federal Police, Hamming said.

It ushered in significant changes in the way Australian police collaborate with international colleagues. It also catalysed the establishment of bomb data centres across South East Asia, where expert knowledge on bomb production could be shared.

"If you want to see some good coming out of it, that was one of the things. That level of cooperation between law enforcement agencies in our region. For the AFP to say, we did something, we learnt something, we did something good," Hamming said

Hamming is among the current and former Australian Federal Police members who are featured on a new podcast, Operation Alliance: 2002 Bali Bombings, which relives the police investigation and the events immediately following the Bali Bombings.

Hamming has shared his experience as part of the oral history of one of the most significant operations in Australian Federal Police history.