Indeed, there is no "Z" fare for travel from any US point I tested to SYD on UA.
Here are some other examples, though:
SFO-JFK, 7 Day advance purchase Z fare is $1808 round trip. CUA fare is $3830.
SFO-LHR, Z fare is $2703 (fare valid from Jun 01-Aug 31). D fare was $9876.