Melbourne's ninth archbishop was welcomed at a special mass at St Patrick's Cathedral on Wednesday evening.

In his homily to a packed cathedral, Archbishop Comensoli urged the congregation to "barrack for gospel joy, not just your footy team" and asked them to take "the first step" in proclaiming Christ.

"Here in Melbourne we have a homegrown example of one such first-stepper, Mary 'The Cross' MacKillop," he said.

"You are the church that produced Australia's first saint and as Mary sits eager and expectant to what lay before her, I now join you on this same threshold, poised in anticipation of what we can do.

"May I claim a newbie's boldness and remind you what I already know about you - you are a church that can produce great fruit. You have it in you already to do this, for you are saint whisperers here."

Archbishop Comensoli's mass was the largest gathering of bishops in Australia ever, according to the Catholic Archdiocese of Melbourne.

The crowd applauded after Pope Francis' letter of the archbishop's appointment was read out early in proceedings.

A former NSW bishop, he replaces Denis Hart, who is retiring.

Australian Associated Press