Anthony Albanese was among those on hand on Tuesday evening for the opening of the memorial's atrium and Anzac Hall, a gallery that focuses mainly on Australia's commitments to the Middle East, Afghanistan and peacekeeping operations.
Retiring chief of the defence force David Johnston and the next leader of the Australian military, current chief of Navy Mark Hammond, were among other dignitaries.
Mr Albanese told those gathered they were bonded by the power of "lest we forget."
"That most unadorned of sentences that dwells within us like a heartbeat – we vow to keep the flame of memory burning so brightly that its glow reaches future generations," he said.
"Today we adjourned the parliament so that everyone who wished to could come mark the opening of this Atrium and Anzac Hall, an addition to the Australian War Memorial that makes that flame burns so much brighter.
"What a sublime and powerful addition it is. A bold vision turned into a reality that enhances the institution of which it is now part."
The prime minister implored attendees to "read all the words" about those who have gone before them.
"Look at the faces and get lost amid the smiles, the hope and camaraderie – the counterpoint to war's relentless, inhuman arithmetic. They are its true cost," he said.
"Yet, amid this loss and sacrifice, what pulses so powerfully is life, and an abiding sense of what is worth fighting for."
Victoria Cross recipient Dan Keighran, who served on the board for a number of years, said the upgrade was great to see.
"It's looking absolutely fantastic," he told reporters.
All living recipients of Australia's highest military honour were invited to the event, with alleged war criminal Ben Roberts-Smith notably absent.
Despite having his bail varied to attend the event in Canberra, the former special forces soldier did not travel to the nation's capital after he fell ill, Sydney's Downing Centre Local Court was told earlier on Tuesday.
"Mr Roberts-Smith felt ill in the last couple days and hasn't travelled to Canberra today to attend the official opening at the war memorial," defence lawyer Slade Howell said.
The decorated soldier is accused of murdering or ordering the murders of five unarmed detainees while deployed in Afghanistan between 2009 and 2012.
He has strongly rejected the war crime allegations and has vowed to fight them in court.
One Nation leader Pauline Hanson, who was among politicians from across the spectrum attending the event, said it was disappointing Roberts-Smith couldn't attend.
"He should have been here," she said.
"He may have been charged, he hasn't been convicted. He's still an innocent man."
Veterans' Affairs Minister Matt Keogh, whose own relatives are listed on the roll of honour, said the redevelopment was an important way to share the stories of Australians who have served the nation.
"Behind every name was a real person and a family," he said.
"Having a war memorial that we can bring our families to, that veterans can bring their families to... is incredibly important."
Addressing the event, Governor-General Sam Mostyn said the new galleries formed a "mosaic of memory" and presented history with "context and candour".
Anzac Hall and the memorial's revamp will add more than 5000 square metres of gallery space - almost a full football pitch's worth - when completed in 2028, totalling 15,000 square metres.
More than $550 million has been spent on the redevelopment.
The memorial has consistently defended displaying an image of Roberts-Smith, from which the body of a dead man has been cropped out.
A plaque under the image has been adjusted several times to reflect Roberts-Smith's arrest and previous civil court cases.
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