Federal Greens Senator Sarah Hanson-Young wrote a reference attesting to the "good character" of a long-time friend and lobbyist who slapped his wife and avoided a conviction in a Canberra court this week.

The man was charged with common assault in December last year after an incident at his home with his wife.

On Tuesday he appeared in the ACT Magistrate's Court where he pleaded guilty to the assault and received a good behaviour non-conviction order.

In the letter to the court, seen by the Herald, Ms Hanson-Young said her friend of 10 years was "one of the key people" who advised her to undertake her recent successful litigation against former Senator David Leyonhjelm after "years of hostile sexist bullying".

"It is true to say that without [his] care and advice over the past few years I would not have been as confident and supported in taking on the sexist and abusive behaviour I have been subjected to as a woman in the Australian parliament," she wrote.