http://www.naa.gov.au/collection/fact-sheets/fs89.aspx
Births, deaths and marriages – Fact sheet 89
The registration of births, deaths and marriages in Australia was initially the responsibility of the colonies and later, the states and territories. Inquiries about births, deaths and marriages within Australia should, in most cases, be directed to the registrar in the state or territory in which the event took place. The table that follows indicates where registers for the Australian states and territories are held.JM comments:
So, for example this fantastic Fact Sheet prepared by the National Archives of Australia, shows that births/deaths/marriages in what is now the Australian Capital Territory would be included in the NSW BDM registry until 1930.
This would be simply because at Federation (1901) the colonies of Victoria and New South Wales had not yet come to an agreement over exactly where the new capital of Australia would be founded.
The Federal Capital Territory (FCT) was established by Commonwealth legislation, thus the design/surveying and building of the capital then needed to occur, and then Parliament was opened, and the administrative bodies for the internal operations of the FCT, which became the ACT, needed to be established, etc.