A hostage standoff was triggered in a Sydney suburb today when a man entered an office complex accompanied by his daughter, demanded to see a person he believed to work there, and told the receptionist that he had a "bomb in his backpack"

The man, reportedly aged in his 50s, was accompanied by a young girl, who reportedly looked about 11-years-old.

She was heard to refer to him as "Dad", when he threw a book at the reception area.

He then took her to the front of the building and closed the door.

Assistant Police Commissioner Dennis Clifford said that police were "very concerned" about the man's comments.

"Until we have possession of the backpack we will remain very concerned about it. There are a number of specialist units in there. We remained very concerned about [the girl's] safety," the Sydney Morning Herald quoted Clifford, as saying.

Appealing for the man to end the siege, he said: "Please work with the police negotiators, we're not here to harm you."

"We are desperately trying to reach a peaceful resolution and whatever the issues are that the man is discussing with our negotiators, I'm sure we can work through them," he added.

The heart of Parramatta remains shut down due to the hostage drama.