Monis letter had a key clue: terror expert

A counter-terrorism expert believes authorities overlooked a clue in gunman Man Haron Monis's correspondence to Attorney-General George Brandis.

Authorities have come under fire for not taking action when Monis wrote to Senator Brandis asking whether it was legal to contact Islamic State leader Caliph Ibrahim aka Abu Bakr al-Baghdadi, two months before the Lindt cafe siege in Sydney.

Monash University professor Greg Barton told ABC radio on Friday use of the term Caliph strongly hinted at Monis's conversion from an Iranian Shi'ite Muslim to a Sunni fundamentalist.