People being held hostage in the Sydney Lindt cafe siege by Muslim fundamentalist Man Haron Monis should not succumb to fear, the survivor to a similar ordeal has said.

Mark Henderson, who was held captive by left-wing Marxist guerrillas for 100 days in Colombia in 2003, also said people who fled the Sydney café into the arms of Sydney's armed police "put their lives in danger."

Up to 15 people remain trapped inside the Lindt café, around 18 hours after Monis – also known as Sheik Haron – sparked the incident, which has shut down a large part of Sydney's government district and Sydney's biggest-ever armed-response operation.

Speaking to Sky News, Henderson said the captives must control feelings of "terror and anger, otherwise it gets worse and worse."

"You are completely powerless, everything you took for granted is gone, you have no freedom," he said.

"The one thing I would say is to stay calm, to try to understand what is going on around you."

During the siege, five captives have managed to escape from Monis – who brought an Islamic flag with him to show TV cameras.

It has also emerged Monis was charged as an accessory in the murder of his former wife and has faced charges for a string of violent offences.

Henderson said fleeing the Lindt café was an extremely risky thing to do.

"It takes a certain kind of person who could do that [escape] and the situation has to be right. You are putting your life in danger."

Henderson was held captive in the Colombian jungle after fighters from the ELN Marxist group captured him in 2003. During the ordeal, he attempted to build bridges with his captors and later made a documentary with one of them.

The Sydney café siege is ongoing.