AUKUS said in a joint statement that delivery of the vehicles will start in 2027.
The program will improve the three nations' reconnaissance and strike capabilities, "and bolster superiority in anti-submarine and anti-surface warfare, mine countermeasures, electronic warfare, and contested littoral manoeuvre," the statement added.
The program comes under AUKUS' so-called Pillar Two to develop advanced defence technology including quantum computing, undersea, hypersonic, artificial intelligence and cyber technology.
"The signature project will deliver a suite of highly adaptable multi-mission UUV payloads designed to support undersea operations and maintain our collective advantage in the maritime domain," Hegseth told reporters in Singapore on Saturday.
Formed by the three countries in 2021, AUKUS is part of their efforts to push back against China's growing power in the Indo-Pacific region.
China has called the AUKUS pact dangerous and warned it could spur a regional arms race.
"This will rapidly give our forces the very most advanced battlefield technologies as together we produce a range of cutting-edge sensors and weapons systems for undersea drones," said Britain's Defence Secretary John Healey.
Healey added the unmanned undersea vehicles will sharpen all three countries' ability to respond to threats, including those targeting underwater cables and pipelines.
"For too long in AUKUS, we talked too much and delivered too little," said Healey, who was talking alongside Hegseth and Australia's Defence Minister Richard Marles on the sidelines of the Shangri-La Dialogue.
Under AUKUS, Australia has been promised at least three Virginia-class submarines from the US in the early 2030s, before a new fleet of vessels is built for delivery from the 2040s.
Australia eventually plans to build five submarines in South Australia, using US technology and parts made in Britain.
with AAP
