A man accused of sending abusive letters to the families of Australian soldiers killed in Afghanistan has formally pleaded guilty in a Sydney court.

Man Monis, who also uses the name Sheik Haron, sent the letters between November 2007 and August 2009.

A court has previously heard the letters criticised Australia's involvement in Afghanistan and labelled the soldiers murderers.

Monis sent letters to the families of seven soldiers killed in action, as well as one man who died in the 2009 Marriott Hotel bombing in Indonesia.

Bree Till received a letter in March 2009, less than a fortnight after her husband Brett died in Southern Afghanistan.

It opened with condolences, before becoming abusive.

"This man accusing my husband of being a child killer whilst dictating how I should raise my children," she said outside court today.

"The fact that there was any question as to whether this was right or wrong, that was difficult."

Monis has pleaded guilty to 12 counts of using a postal service to offend on the grounds of recklessness.

His co-accused, Amirah Droudis, has also pleaded guilty to one count of aiding and abetting Monis, after she sent an item of mail in May 2008.

Monis gained notoriety by chaining himself to a railing outside a Sydney court in 2009 in protest against the charges he was facing.

In February, he also lost a High Court challenge to the charges, after claiming they were unconstitutional.

The case had been seen as an important test of the implied right to freedom of political speech in the Constitution.

Monis left court today with two fingers in the air, signifying the peace sign.