Pressure on New South Wales opposition leader John Robertson to resign following the revelation he signed a letter on behalf of Sydney siege gunman Man Haron Monis, to help him get access to his sons.

Australian media reports that some MPs are counting numbers against Mr Robertson in an attempt to get a special caucus meeting called to settle the leadership.

Mr Robertson denies he endorsed Monis, saying it was a straightforward matter, similar to the thousands of constituent requests handled by politicians every year.