Among them was British-born actor Miriam Margolyes, best known for playing Professor Sprout in the Harry Potter movies, and for her role in caustic British comedy Blackadder.

Ms Margolyes, who first came to Australia in the 1980s and lives in the Southern Highlands, was asked why it had taken her so long to take citizenship.

''Well, I think you have to be sure, don't you,'' Ms Margolyes deadpanned. ''And now I am.''

To howls of laughter, Ms Margolyes told the crowd at Canberra's Commonwealth Park that she came to Australia because, ''I fell in love with an Australian … woman, by the way, in case anyone didn't know I'm a dyke - and I'm very happy.''

Also becoming a citizen was British-born Ian Mears who, wearing a ''Kiss me, I'm Aussie'' novelty tie, persuaded Prime Minister Julia Gillard to oblige with a good-humoured peck on the cheek.

Mr Mears said he wanted to become an Australian citizen so he could vote and do jury duty: ''Hopefully a good, fun, case.''

Ms Gillard told the gathering that since Ben Chifley hosted the first citizenship day ceremony in 1949, more than 4 million people had sworn their loyalty to Australia.

''And I am proud to be one of those 4 million,'' she said. ''As migrants, we did not just adopt a new country. We helped transform and enrich it. And together, as new Australians and old, we made this experiment work.''

In Adelaide, Opposition Leader Tony Abbott offered a message of empathy to those feeling anxious about change.

Mr Abbott said it should be in accordance with Australia's customs and traditions.

''Sometimes, those of us who have been here for a long time get a little bit anxious about the changes that are taking place in our country,'' Mr Abbott told an Australia Day breakfast and citizenship ceremony.

''It should be change in accordance with the customs and traditions of our people.''

Mr Abbott said he cherished the cultural diversity of Australia.

''Above all else I cherish our unity,'' he added.

''And by looking to become Australian citizens, you are celebrating that which we have in common.''

with AAP