The Air Services Treaty between the United States and Australia doesn't list US-Tahiti as an allowed route permitted to be offered by carriers designated by the government of Australia.

https://www.state.gov/documents/organization/168386.pdf
See Annex I at the back of the Treaty for the list of permissible routes. In particular, a carrier must only offer a service if it touches its own territory.

(It is important to note that codeshare flights are held to the same standard as prime flights.)

Therefore there is no basis in law for QF to sell this route.