A republic is about trusting the people. Australians should have a say in who is our head of state - it should not be decided by someone being born in England.
Replacing the King or Queen of England and the Governor-General with an Australian would give us a proper head of state who is above politics. We would have a dignified, prestigious Australian head of state - a person to represent our whole nation who is separate from the political parties and the Parliament.
Our democracy belongs to Australians, so instead of the top job belonging to the English Queen (or King), it makes more sense to have an Aussie head of state to represent us.