I have a US non-provisional that claims priority to a provisional one. It is not published yet but its priority period has expired. I also have a pending Australian provisional application for exactly the same invention. Can I file a PCT application claiming the priority date only of the Australian provisional which is the first application in this country but has a later filing date than the US one?
No. Under the Paris Convention you can only claim priority to the first filed application. The exception to this is if the first application is abandoned without being published and was never used in a priority claim, then the second application in that same country can be used for priority. Article 4 is very clear.
Article 4 C (4) A subsequent application concerning the same subject as a previous first application within the meaning of paragraph (2), above, filed in the same country of the Union. shall be considered as the first application, of which the filing date shall be the starting point of the period of priority, if, at the time of filing the subsequent application, the said previous application has been withdrawn, abandoned, or refused, without having been laid open to public inspection and without leaving any rights outstanding, and if it has not yet served as a basis for claiming a right of priority. The previous application may not thereafter serve as a basis for claiming a right of priority.
If I may summarize the timeline:
02 April 2017 - US Provisional(Not published)
01 April 2018 - US Non-Provisional, converted from US Provisional (Not published)
29 April 2018 - Australian Provisional (Not published)
24 May 2018 - Day you asked the question
Can you file a PCT claiming the priority date of the Australian Provisional?
If you have not made any public disclosure, the answer is yes.
The applicable rule here:
Article 4.C(4) of the Paris Convention dictates that the 12-month window for filing a PCT application commences on the filing date of the earliest application in the family.
In other words, you cannot file a PCT application based on the priority of your US Provisional because the 12-month window has passed.
However, you can still file a PCT application based on the priority of your Australian Provisional since you are still within the 12-month window.
Note that you will need to withdraw your US Non-Provisional to prevent the novelty of your subject matter in your PCT Application from being negated by the US Non-Provisional when it publishes.
This link provides more information: https://www.smithhopen.com/PCT_filing_deadlines_and_priority_dates.aspx