At least five firefighters — three in New South Wales and two in the state of Victoria — had already been killed this fire season. The overall death toll from the bush fires now exceeds 30. More than 2,500 homes have been destroyed, and millions of acres have burned.

American firefighters have been heavily involved since at least early December. But as the blazes have intensified, dozens more have arrived to help, extending a history of firefighting collaboration between the two countries that goes back almost two decades.

In 2018, as California was in the grip of its worst fire season ever, more than 130 Australian and New Zealand wildfire experts traveled there and to the Pacific Northwest to battle blazes, according to the National Interagency Fire Center.

Since December, about 200 American fire personnel have been deployed to Australia, mostly in New South Wales and Victoria, where the crisis has been centered.

Some of the visiting firefighters have been part of elite ground crews, but far more have arrived with expertise in fire management, planning and aviation.

Firefighting aircraft have become a major priority in Australia. Officials started with plans for only a handful of leased tankers, but as pressure on Australia’s volunteer firefighters has intensified, the government has increased the size of its rented fleet.