The head of the US Navy's Indo-Pacific Command is urging Congress to build a new missile defence base in the Pacific as part of a larger strategy to counter China's military threat to the region.

The Pentagon is seeking $US77 million ($98 million) to build a permanent "land-based integrated air and missile defence system and associated weapon delivery system on Guam", as outlined in the Indo-Pacific Deterrence Act.

In addition to the defence system, which is designed to track down and destroy missiles before they do harm, the US is considering an increase to its ground-based missiles on Guam that could fire on targets over 500 kilometres away.

Almost a third of Guam's land is controlled by the US military, and Ms McManus fears its presence is destroying the island's natural environment.

The Biden Administration will likely be "focussed on national securities," meaning Australia may have to make harder choices in the defence space, according to the Lowy Institute’s Richard McGregor.

“Trump was focussed on trade,” Mr McGregor told Sky News. “The Biden Administration might be much more focussed on national securities.

“That might mean harder choices for us in the defence space.”

Mr McGregor said this may mean the US would expect Australia to look at things including “intermediate range missiles” and the possible placement of “more Marines in Darwin”

“They might like us to do greater naval exercises in the South China Sea,” he said. “I think they’re going to expect us to step up as a strong ally.”

The U.S. Marines with 3rd Battalion, 12th Marine Regiment, a part of Marine Rotational Force-Darwin (MRF-D), fired Guided Multiple Launch Rocket System munitions, known as GMLRS, during Southern Reach, at Bradshaw Field Training Area, Northern Territory, Australia on August 15, 2019.

The HIMARS Marines fired two GMLRS in Australia for the first time ever, according to Cpl. Kallahan Morris. The HIMARS is a weapons system made up of the M142, five-ton chassis vehicle and can carry either a launcher pod of six rockets or one MGM-140 Army Tactical Missile System (ATACMS).

The live-fire training conducted was over an area of 3,282 square miles.

HIMARS is participating in MRF-D for the first time. These Marines and equipment provide MRF-D an extended range precision strike capability that can further shape the battlespace.

the HIMARS Marines with MRF-D conducted multiple fire missions, shot two GMLRS rounds, and enjoyed training in the heat of Australia’s Northern Territory dry-season.