Mark Latham has tweeted his 'unreserved apology' over comments accusing university student Mohamed Nizamdeen of "plotting to kill senior federal MPs" in the midst of a defamation suit between the pair.

The University of NSW PhD student, 26, is suing Mr Latham in the Federal Court over multiple tweets posted after Mr Nizamdeen was charged with terrorism offences following the discovery of a notebook, accusations which have now been dropped.

Police withdrew a terror charge against UNSW PhD student Mohamed Nizamdeen.

One of the tweets accused Mr Nizamdeen of "plotting to kill senior federal MPs and blow up Sydney Opera House and police/train stations", while another urged people to "try avoiding" the student "and his mates".

The former Labor leader asserted through a newly-appointed lawyer before court yesterday that the accusation was his "honest opinion" and caused little harm.