Celebrations have begun in Rome and Australia after the announcement of Australia's first Roman Catholic saint, Sister Mary MacKillop.

Pope Benedict the 16th has announced the canonisation at a meeting of cardinals at the Vatican. The official ceremony will be held on the October 17.

It has been a long road to sainthood for the Melbourne-born nun, with calls for her beatification starting in the 1920s.

The announcement brought excitement and relief for Sister Maria Casey.

Calls for Blessed Mary's beatification started even before Sister Maria was born.

But the Sister of St Joseph has become one of the loudest voices of the campaign, as the postulator for Mary MacKillop's cause.

"The announcement was made in the middle of a formal prayer session, so it was a very subdued church-like environment, if you like," she said.

"So we weren't allowed to cheer and stamp and clap or anything like that."

Sister Maria says the fact the sainthood has been 85 years in the making, demonstrates just how thorough the process is.

"It shows that it's a process with integrity because it wasn't rushed; it wasn't hurried," she said.

In 1861, Blessed Mary was credited with curing a woman with terminal leukaemia, and last year the Vatican decreed she has cured a woman with inoperable cancer, giving her the second miracle she needed for canonisation.

In the meantime though, says Sister Maria, there were medical documents to chase down and testimonies to record.

"[We had to wait] for scientific experts to examine them and to come to a conclusion whether the cure could be explained by scientific means," she said.

"And then after that the theologians also had to do their search.

"So this all has happened in the last two years, which is remarkably quick in Roman times."

Determined and dedicated

Mary MacKillop died just over a century ago, and is best remembered for her work with children and the needy.

She opened her first school in a disused stable at the age of 24.

Australian Ambassador to the Holy See, Tim Fischer, says her work laid the foundations for education of the disadvantaged in many country areas.

"Mary MacKillop flogged herself around outback Australia; lived for periods in places like Numurka and Penola, Coonawarra South Australia," he said.

And he says she bravely fought her excommunication in 1871 when she fell out with other members of the clergy over educational practices.

"Certainly, she stood determined and dedicated and, as Rome ruled, as the Holy See of the Vatican ruled - the Popes of yesteryear ruled - she was correct.

"It would make a great plot for an Italian grand opera, dare I say."

He says the canonisation in October will be a big event and will include a vigil concert and service, the actual canonisation on the Sunday and the subsequent follow up on the Monday.

Mr Fischer says visitors can expect lots of tourism vendors setting up around October with badges and tea towels depicting the newly canonised saint.

"Rome will be Rome," he said.

The Catholic Archbishop of Sydney, Cardinal George Pell, has encouraged young Australian catholics to travel to Rome for the canonisation of Mary Mackillop.

"Mary MacKillop [has] got very important lessons for young Australians, the usefulness of faith and goodness and hard work and service," he said.

"If you see a need, try and do something about it."

'A practical person'

Historian and Catholic Commentator Paul Collins told ABC1's Lateline she recognised and then met the needs of others.

"I have described her as a great Australian sheila," he said.

"Now, that's perhaps a rather crude phrase, but nevertheless, I think it sums up the type of person that she is.

"A practical person, a person who saw a real social need, who saw the children in the bush particularly needed education and culture and she organised a group of sisters to be able to meet that need."

Sister Maria Casey says the canonisation will mean very different things for different people.

"For Catholics this will mean that the church has recognised that Mary was a holy woman and she died, but she was a saint," she said.

Catholics in the South Australian town of Penola are expecting an influx of visitors interested in Mary MacKillop's legacy, with church bells marking her upcoming canonisation.

In 1866, the young teacher set up a school and the Sisters of Saint Joseph in the country town in the South Australian south east.

Claire Larkin from the Mary MacKillop Penola Centre says Penola is where Mary MacKillop's life's work began.

"Mary wasn't here for a long time," she said.

"But this is where she met Father Woods, this is where they had the dream, this is where they founded that order and the Sisters of Saint Joseph all look on Penola as being the beginning.

"It's the type of place that Mary and the Sisters of Saint Joseph went to, they were outback and remote places and they brought education to the poor and isolated children."