Australia and Fiji signed a major defense alliance on Monday, committing each country to come to the other's aid if either is attacked, as China seeks to expand its influence in the region.

The Ocean of Peace Alliance marks Fiji's first-ever alliance, while the Pacific nation will become Australia's fourth formal ally, after the US, New Zealand and Papua New Guinea.

"The Ocean of Peace Alliance introduces mutual defense obligations, and there's no higher obligation than to come to each other's aid at a time of need," Australian Prime Minister Anthony Albanese said in a joint news conference with his counterpart Sitiveni Rabuka in the Fijian capital Suva.

"An attack on Fiji from an outside force would trigger Australia's full support for Fiji and for its sovereignty," Albanese said.

The defense pact comes as China expands its economic influence and deepens security ties in the Pacific. Australia has said it is in a "permanent state of contest" with China in the region and has sought to curb Beijing's influence through security and defense deals with Pacific nations.

Australia signed a security agreement with Vanuatu last week, under which it will remain the country's preferred security and policing partner.

The Ocean of Peace Alliance requires both countries to consult on security threats and to "act to meet the common danger" if either is attacked, similar to a defense treaty Australia signed with Papua New Guinea last year.

The treaty text said other Pacific countries could be invited to join if they can "further the purposes and principles" of the treaty and contribute to the security of the Pacific.

New Zealand would be interested in joining the agreement, New Zealand Prime Minister Christopher Luxon said on Monday.

Australia, Fiji also expand economic, security cooperation

Australia and Fiji also signed the Vuvale Union, expanding economic and security cooperation between the two countries. It would be backed by A$1 billion ($692.7 million) in Australian investment over the next decade, Albanese said.

"Combined, these treaties represent one of the most significant endeavors Australia has ever undertaken in our history with any country," Albanese said.

Rabuka said he did not expect China to object to the agreements, despite Beijing's previous criticism of Australian security deals in the Pacific.

"It does not threaten Fiji's relationship with China nor Australia's relationship with China," Rabuka said.

"We have made it very clear in the past, your enemies are not necessarily my enemies."

Albanese will visit the Solomon Islands on Tuesday to meet his counterpart, Matthew Wale, and is expected to continue discussions on a comprehensive strategic treaty.

Australia calls China's planned missile test 'destabilizing' for Pacific region

The agreement's announcement comes as China prepares to test fire a nuclear-capable ballistic missile with a dummy warhead in the South Pacific in the next 24 hours, Australian media reported on Monday, adding that regional diplomats have been notified of the launch.

The Chinese embassy in Australia did not immediately respond to a request for comment.

Australian Foreign Minister Penny Wong said on Monday that China had notified the government of plans to conduct a sea-based missile test into the Pacific, describing the launch as "destabilizing" to the region.

"Australia has been clear that this proposed test is in the context of a rapid military buildup by China, which is lacking in the transparency and reassurance as to intent that the region expects," Wong told reporters at a news conference in the Fijian capital Suva.