AUKUS Undersea Defense Act This bill authorizes the Department of Defense (DOD) to sell Australia up to two Virginia-class attack submarines under the Australia-United Kingdom-United States security partnership agreement (AUKUS). The bill authorizes DOD to transfer the submarines from existing Navy inventories. The bill also applies existing law to the transfer under this bill of special nuclear material related to submarine reactors, authorizes the Navy to determine the appropriate shipyard in any of the AUKUS countries for repair of U.S. Navy submarines, establishes a submarine security activities account to accept payments from Australia for non-nuclear related aspects of submarine security activities, and authorizes the President to transfer defense services to Australia to develop the Australian submarine industrial base for AUKUS activities.