A leading British terrorism expert says he would have been "screaming out" for a pre-emptive plan for NSW police to storm the Lindt Cafe during the Sydney siege if he was in charge.
Detective Senior Constable Simon Chesterman, the head of armed policing in the UK with special responsibility for siege management, said there was "no rational basis" for not approving a direct action (DA) plan.
"I would have been absolutely screaming out for a DA to be in place to give me that option," he told an inquest into the deadly 2014 siege on Thursday.
"I would expect and demand that a DA be in place."
Commanders twice refused to approve a direct action plan produced during the Martin Place stand-off.
Det Sen Const Chesterman said the plan appeared to be workable, and was inherently less risky than an emergency action plan, which was ultimately triggered after Monis executed hostage Tori Johnson.
A direct action plan was more detailed, could be rehearsed, and could be executed at a time chosen by police, he said.
The belief of police that Monis was carrying an improvised explosive device made the direct action plan an even more attractive option.
"The longer you leave him in a stronghold with hostages and an IED the more chance you've got of him detonating it," he said.
Det Sen Const Chesterman, who led a UK review into the handling of the siege by NSW officers, said the direct action should have been approved but not necessarily initiated.
"Initiating was still a very difficult decision bearing in mind the risk," he said.
He said the initial police response of "contain and negotiate" was justifiable because Monis, unlike other Islamic State terrorists, had not caused mass casualties.
But police should have had in the back of their mind that at some point he was going to start killing people.
"When you are dealing with potentially Islamic terrorism in the form of a siege, it is not going to end well," Det Sen Const Chesterman said.
As commander, he would've become increasingly frustrated with the lack of contact with Monis by the time the stand-off had dragged into its fifth or sixth hour, and pushed negotiators to employ different tactics.
By the time Monis began separating hostages and instructing them to phone home, the threat had "gone through the roof", he said.
Det Sen Const Chesterman said it's likely he would have ordered police to assault after Monis first shot at escaping hostages at 2.03am on December 16, 2014.
Had he not triggered the emergency action then, he definitely would have minutes later when Mr Johnson was forced to his knees.
The inquest has heard snipers reported seeing Mr Johnson kneeling at 2.06am, but radio failures meant this information never reached commanders.
"Without a doubt the kneeling hostage was the tipping point to be met and I would expect the EA to be initiated at that stage if not before," Det Sen Const Chesterman said.
Officers ultimately stormed the cafe and killed Monis at 2.14am after Mr Johnson was shot dead, with hostage Katrina Dawson also fatally wounded in the crossfire.
The inquest continues in Sydney.
