SYDNEY (AFP) - The Sydney cafe at the centre of a deadly siege has been vandalised with police investigating on Wednesday after rocks were thrown through five of its windows.

Witnesses told police they saw a man loitering near the upmarket Lindt cafe in the city's financial heart at the time of the attack in the early hours.

"We believe a man in his 40s with facial hair and tanned skin may be able to assist with inquiries," a New South Wales police spokeswoman said.

They added that it was too early to say whether it was a targeted or random attack, which coincided with an inquest into the siege underway nearby.

Iranian-born Man Haron Monis, a self-styled cleric, took more than a dozen customers and staff hostage at the cafe on Dec 15.

The 17-hour standoff ended with the deaths of two hostages and the gunman in an ordeal that sparked fears about national security and caused an outpouring of grief.

The inquest, which started last week, is probing Monis' motivations, including whether he was a "lone wolf" carrying out a terrorist act inspired by the Islamic State in Iraq and Syria (ISIS) or a deranged individual.

The inquest is set to continue throughout the year with an outcome not expected until 2016.

The cafe was badly damaged during the standoff and only reopened in March with two gold plaques erected at the entrance in honour of the victims - manager Tori Johnson and barrister Katrina Dawson.