SINGAPORE — Australia’s defense minister, Stephen Smith, says the government has made a formal decision to buy a fifth Boeing C-17.

“We will acquire another C-17 heavy-lift plane and we will do that as quickly as we can,” the minister told the Australian Broadcasting Corp.

Australia already has four C-17s, one of which is now in Japan. Smith says Australia and the U.S. are the only two foreign countries with military aircraft in Japan at the moment.

“We’ve been using the C-17 . . . in conjunction with the Japanese authorities to transport both equipment, fresh water for example, but also Japanese emergency workers themselves and their equipment and vehicles, and we’ll be doing that for the rest of the week until at least the weekend,” he says. “Then we’ll make a judgment about whether to continue.” The C-17 is “proving invaluable at the moment,” he adds.

A spokesman for the minister says the purchase of the fifth C-17 “has been decided by cabinet ministers” and “in the next few weeks details will be fleshed out,” such as the delivery date.

The Royal Australian Air Force also used its C-17s to help with New Zealand’s disaster relief efforts in Christchurch, where a 6.3-magnitude earthquake shook the city on Feb. 22, destroying buildings and killing more than 100 people.

Smith disclosed at Australia’s Avalon Airshow on March 2 that the country was considering buying a fifth C-17. At the time, he said “an acquisition of an additional C-17 would almost certainly obviate any need for the acquisition of two additional C-130 aircraft, currently planned for after 2013-14” (Aerospace DAILY, March 2).