Police decided not to storm the Lindt cafe after being told by experts the blast from a potential bomb inside gunman Man Haron Monis's backpack could reach 150m.
Officers only later realised Monis in fact did not have a bomb in his backpack once the siege came to its fatal end in Sydney's CBD.
Nonetheless, the possibility that Monis's claim that he was carrying a bomb was true kept police from storming the cafe.
One of Monis's hostages, Marcia Mikhael, has criticised the police handling of the December siege, saying they were too slow to intervene.
Ms Mikhael, 43, was one of 18 people held hostage by Monis for more than 17 hours with the siege ending in the deaths of cafe manager Tori Johnson, barrister Katrina Dawson and Monis.
Ms Mikhael, who was shot in the leg during the siege, told the Seven Network on Sunday the army would have handled the situation better than the police.
But a source who was involved in the police response said officers held real fears about the contents of Monis's backpack.
"No one talks about it now because obviously it wasn't (a bomb)," he told AAP.
"But right up until even just after it happened, the bomb squad was there thinking this guy still had a bomb.
"The police that went in there were expecting it to explode."
The source, who asked not to be identified, said based on how the bag looked, police thought it contained a 4kg bomb.
"They did the calculations and they said that if that explodes it would blow from Hunter Street to King Street (a diameter of 300m) - so they had to bear that in mind."
He says officers involved in the operation, who are unable to comment on the siege until a coroner's report is completed, would be disappointed by Ms Mikhael's claims.
"I think a lot of police are probably a bit upset," he said.
He said the police's Tactical Operations Unit, who handled the operation, are trained specifically for sieges and are the "best of the best".
NSW Police Assistant Commissioner Frank Mennilli defended police actions.
"At the end of the day, I have a particular theory that police are asked to make a split-second decision and then others spend weeks and months dissecting that decision," Mr Mennilli said.
"Police are going out there from day to day putting their lives at risk to protect the community."
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