MySQL Fast Database Data Validation Master/Slave
The Master is now in the US?  But will later be in Australia?

Rather than simply have Master (in US) + Slave, also set up the binlog (etc) so that it is Master-Master.  However, continue to write only to the US Master.

Once you have spent days on validation, you should be ready to point the clients at the Australia Master.  At this point, you have to trust that the days of replication makes across the big pond correctly.  (You could spend more days re-validating, but I expect it will say OK.)

I don't know for a fact, but I would expect pt-table-checksum to work almost as fast in a live system as an idle one.