NSW Police were right to try to negotiate with the gunman, according to a former senior police officer familiar with counter terrorism training.

"There is a very strong history that negotiations work and time does solve problems," said the former officer, who asked not to be named.

"The intention would have been to negotiate with him, to try to influence him.

"The longer it goes, the more chance you have got, there is decades of evidence  that is the best way.

"Unfortunately, in this case the risk rose dramatically and unexpectedly."

He said police involved in the Lindt cafe siege "train, train, train" and would have had a number of contingency plans in place "shortly after" it started.

The first would have been "to do something quickly in response to a major threat".

"I would say from what I have seen (that) plan had to be activated, they have obviously decided to go because of something occurring.

"That something occurring was a shot being fired and the grave danger against the hostages."

He said if the gunman had in fact shot and killed a hostage, police had no choice but to go in.

Asked about a number of scenarios, including whether officers could have gone in though the roof to end the siege earlier, he said: "It depends on the access."

It was more likely they had a plan to enter covertly, perhaps though a fire exit or a back door.

Former NSW assistant commissioner Clive Small said going though the roof would "create a racket" and was not really an option.

"You are exposing yourself to gunfire and you're not in a position to fire back because you don't know where the hostages are."

Both Mr Small and the former officer were skeptical about theories the gunman could have been shot through the window.

Both said it appeared the gunman had shown himself early in the day, before snipers would have been in position and also before police were in possession of vital information, such as how many gunmen were in the café.

"It's a very dangerous thing to do, for a start you don't know what's going to happen to the projectile once it goes though the glass," said the former senior officer.

Asked who would have made the decision to go in, he said "the office of constable gives anyone (in the force) the power to make a decision if they feel a life is being threatened.

"More likely than not, it would have been a supervisor."

The supervisor could have been a member of the heavily armed team or could have been a short distance away communicating by radio.

Information gleaned from the escaped hostages would have also informed the thinking of police, as would the criminal history of the gunman, Man Haron Monis.

There may have also been electronic evidence via a listening device that also informed the decision.

Mr Small said he thought it was significant the gunman "had no escape route."

"It seems it was quite clear he had no plan on leaving there alive."

The former officer said the police involved were "very disciplined".

"They are as good as anyone in Australia, or the world for that matter."

Another former NSW assistant police commissioner Ken McKay, who retired last year after 35 years in the service, said it was evident the strategy had been to "wait it out" and monitor events inside through observation and negotiation.

Mr McKay, who headed the first NSW Police Middle Eastern Organised crime Squad, said: "The main aim is to get everyone out safely – and a lot of the time, that includes the offender."  But he added that when gunfire suddenly sounded inside the cafe, "that immediately changes the game." In that instance…when police are suddenly forced to go from negotiating and containing to attack…everything becomes different. With that, comes risk."