They're the seven, unaccounted-for minutes of confusion when the Sydney siege escalated from an apparent waiting game to a full-scale tactical assault on the Lindt Chocolat Cafe.

It was at 2.03am on Tuesday when those gathered around Martin Place heard a "very loud bang" ring out around the darkened, roped-off square.

Sydney Morning Herald photographer Andrew Meares, who had been keeping watch at the location for hours, knows that was the time, because it's stamped on the photographs he began snapping on his camera as soon as he heard the commotion.

"There was talk that something big was happening," said Meares.

But it wasn't until seven minutes later that police made their dramatic raid on the cafe.

At 2.10am, Meares heard another, identical loud bang ring out and began snapping more photos.

It was that second bang that appeared to prompt police to storm into the cafe, where gunman Man Haron Monis had taken 18 customers and staff hostage the previous morning.

Meares does not know precisely what the two, identical bangs were.

His first suspicion was that they may have been flash-bang grenades to assist the hostages escape.

On reflection, he believes they may have been Monis discharging his gun, Meares said.

"The pitch, resonance and duration [at 2.03am] was exactly the same as what I heard at 2.10am," Meares said.

"The second bang was followed by a higher-pitched, sharper sound we now know to be a stun grenade. Then there was a massive cacophony of noise. It was frightening, and sickening," he said.

Meares said that, within seconds of hearing the first bang, two male hostages ran from the cafe, "sprinting faster than Usain Bolt".

Seconds later, another group of hostages emerged from the same door and into the arms of waiting police.

Then there were a few minutes of apparent calm, Meares said. A number of officers who had emerged, guns drawn, after the first bang had retreated to their hidden position in the doorway of the Hotel Chambers in Martin Place.

When the second bang came, tactical police moved in within seconds.

Meares captured images of the police entering the cafe, where hostages Katrina Dawson, 38, and Tori Johnson, 34, were shot dead, alongside the 50-year-old gunman.

Police have not yet revealed what happened in the critical minutes when police stormed into the cafe, or whether the hostages were killed by Monis, or were caught up in the crossfire.

Meares said that, after the second bang at 2.10am, it was a further seven minutes, at 2.17am, until an officer flashed a torch from the top of Martin Place.

That signal sent a team of paramedics running up the square with their gurneys to treat the injured.

SMH