Ten years after the Lindt Cafe siege sent shock waves around the nation, the man at the helm of the landmark investigation into the tragedy has delivered a stark warning for Australians.

“People have this idea that the terrorist threat has gone away,” former NSW coroner Michael Barnes said.

“I think at any stage, something like that could occur again.”

Barnes led the inquest into the December 2014 siege, in which terrorist Man Haron Monis took 18 people hostage inside Sydney’s Martin Place for 16 hours.

Monis fatally shot cafe manager Tori Johnson, while barrister Katrina Dawson was killed by stray police bullet fragments in the final moments of the siege.

Now head of the powerful NSW Crime Commission, Barnes handed down a series of findings and recommendations after his 18-month inquest into the tragedy.

He found NSW police did not act quickly enough, the gunman should not have been allowed out on bail at the time of the siege, calls for help from the hostages went unanswered, and victims’ families were treated insensitively.

Although Barnes remained wary of a laissez-faire attitude towards domestic terror threats, he said lessons had been learned from the siege.

“Police are much better placed now to deal with those issues,” he said.

“It was a learning experience for NSW but also police around the country.”

One of the inquest’s major findings was that police missed the chance to storm the cafe after Monis fired the first shot, an opportunity that, if taken, might have saved Johnson and Dawson’s lives.

After the inquest concluded in 2016, then-NSW police commissioner Mick Fuller conceded police should have gone in earlier.

“I give this assurance to everyone … we won’t wait 17 hours again,” he said.

One year on from the deadly siege, Australia’s largest police force adopted a shoot-now strategy for terrorist situations, replacing its controversial contain-and-negotiate procedure, which the inquest found cost valuable time.

At the time of the siege, police were also required to make individual decisions about when to use deadly force.

“People under incredible stress and in very great danger themselves are perhaps not best placed to make those decisions,” Barnes said.

“So we thought putting that a bit higher up the hierarchy was justified.”

NSW laws have now been changed to adhere to the recommendation.

Police resourcing was another concern identified at the Barnes inquest.

With their specially equipped truck off the road, negotiators worked out of a four-wheel drive before moving to a cramped room inside a Leagues’ Club.

The building had no whiteboards or live feeds and only one landline. Meanwhile, the primary negotiator had never handled a hostage situation before.

Key demands by Monis – including securing media coverage, organising a meeting with the prime minister and being given an Islamic State flag – were delayed in reaching top commanders, and none were granted.

“The idea there that you can’t make concessions to a terrorist, I think that did hinder some of the way [police] acted on the day,” Barnes said.

“No concessions doesn’t mean you can’t engage, doesn’t mean you can’t give them anything that they might want if that’s going to de-escalate the situation.”

Negotiation training policies have also since been updated.

The inquest additionally unveiled stark information about how prosecutors and other sworn officers dealt with Monis in the weeks and months preceding the attack.

The lone gunman, who was fatally shot during the siege, was out on bail in 2014 after being charged with being an accessory to murder and more than 40 counts of sexual and indecent assault.

Some victims’ families have expressed disbelief that Monis was allowed in the community while facing such serious matters.

Police preferred the sexual assault charges against him via a court attendance notice while he was at large instead of arresting him.

Barnes found this made it more difficult for prosecutors to argue that he should be taken into custody.

However, the inquest found prosecutors submitted “inadequate” oral oppositions to Monis’ request for bail and no written submissions.

Siege victims’ families, who suffered through hours of torment, were also left in the dark for much of the ordeal.

Barnes found they were gathered together in a hostage room too close to the cafe and were able to hear explosions and gunshots.

“They then didn’t hear anything officially for a number of hours and they were all kept in a large space and could see individual hostages who had been released reunited with their family members,” he said.

“The remaining families diminished in number until it became obvious that there weren’t any more hostages.”

In the siege’s aftermath, a sea of flowers formed on Martin Place, paying tribute to the traumatised hostages and the two fatally wounded victims.

Ten years on, little remains at the site to remember the 15 December siege.

The cafe is now a Dan Murphy’s wine cellar, where a central tasting table is adorned with flowers in memoriam.

A permanent exhibition is embedded into the concrete in Martin Place, with small flowers set into the pavement behind glass frames.

The NSW government will mark the tragedy’s 10th anniversary throughout this week, with commemorative displays and flower laying from Tuesday.