Australian authorities have begun an investigation into the 16-hour siege in the Lindt cafe in Sydney, which resulted in the death of three including the attacker, identified as Iranian refugee Man Haron Monis - as tributes continue to pour in.

Two hostages – Tori Johnson, 34 and Katrina Dawson, 38 – died when special forces operatives stormed the cafe to take out the gunman.

Investigators are looking into the motives of the 50-year-old "lone gunman", who was known to have had extremist leanings and a criminal record.