Hi Guys.
Sorry to come in this one a little late but I do find this topic very interesting.
Early Republicanism in this country was quite robust. A good explaination of this can be found here:
http://www.independentaustralia.net/2011/republic/a-short-history-of-australian-republicanism/
As you can see the very early revolts were mostly Irish Convicts whose only interests were to sieze ships and return to Ireland. They were hardly interested in establishing a Republic or any other form of armed takeover of the colony.
Looking further afield there was a small uprising near Bathurst around the 1820's that led to pitched battles with police and troopers. But they recieved little in the way of public support and sympathy.
The Eureka Rebellion was probably the most likely of scenarios. Indeed any form of Republican foothold on the continent was in Victoria. However the date of the Rebellion is its achillies heel. 1853 was a time of minimal population, resources and infastructure. If the Battle of Eureka was a success to the Rebels its likely they wouldn't have achieved much more than local control around the Ballarat area and little else.
The most likely scenario of an insurgent rise in Republicanism would be the 'Kelly Uprising' of 1878 - 1880. If theorists are correct and Kelly succeeded at Glenrowan there may have been a declaration of the Republic of North East Victoria.
It's widely believed that Kelly had not only sypathisers but an active support network. This is also evidenced by the thousands petitioning for clemency prior to Kelly's execution.
What I'm looking at here is the possible fall of British controlled Victoria and a 'domino effect' of further colonies either severing ties with Britain or falling to Republican control.
Would this be achievable? Considering the Kelly Uprising happened late in the 19th century and closely followed by the Federation movement and with active Republican sentiments at the time I think it would certainly be possible to see the declaration of the Republic of Australia by 1900 and not the Commonwealth of Australia on 1 January 1901.
This is very interesting and I might work on a timeline.