UK defence minister Grant Shapps signed the agreement in Canberra with his counterpart Richard Marles, establishing a legal framework that makes it easier to host troops and share military intelligence.
The agreement stops short of a full mutual defence pact, which would bind one side to intervene if the other was attacked.
But it does include a "commitment to consult" about emerging threats and establishes a "status of forces agreement", which makes it easier to host soldiers from the other nation.
"It is extraordinary, actually, the United Kingdom and Australia didn't already have a defence cooperation treaty in place," Shapps said after the signing ceremony.
Alongside the United States, Australia and the UK are members of the fledgling AUKUS defence alliance -- a landmark pact aimed at curbing Chinese military expansion in the Asia-Pacific.
Barely two years old, there are already signs the AUKUS programme is under threat -- and some fear Donald Trump could scrap it completely if he returns to power next year.
Australian National University security analyst David Andrews said Thursday's agreement gave the stalling AUKUS plan some much-needed momentum.
"If there was a Trump administration at end of year, and for whatever reason they were not interested in pursuing the agreement, or not in the same way it is envisioned now, there is potential for a heavier bilateral pathway," Andrews told AFP.
A major pillar of the AUKUS pact is a promise to help Australia build and acquire a fleet of potent nuclear-powered submarines, one of its biggest-ever military upgrades.
Thursday's agreement would make it easier, for example, for Australian sailors to train on the UK's nuclear subs, or for British crews to be based in Australia.
"This is a reflection of increased engagement between our two defence forces," said Australian defence minister Marles.
"And it will greatly streamline the ability for us to work together."
- Consult on threats -
London and Canberra have pledged to consult each other if looming regional threats start veering towards conflict.
"I think one of the most important elements is it describes a mechanism by which we consult when either of our countries are under threat," said Shapps.
Australia is deeply involved in US-led efforts to counter China's increasingly assertive behaviour in the Asia-Pacific.
Among a host of other initiatives included in the deal is "closer collaboration on undersea warfare", and greater UK contribution to Australian-hosted joint military exercises.
Australia has also agreed to join a coalition with the UK and Latvia that aims to supply drones for the Ukrainian war effort.
The UK has been a major backer of Ukraine in its grinding war against invading Russian forces.
High-level talks are slated to continue on Friday, when UK foreign secretary David Cameron meets Australian foreign minister Penny Wong in Adelaide.
Earlier this week Australia played host to Beijing's top diplomat Wang Yi, who returned to the country for the first time since 2017.
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