Western Australia is welcoming back travellers from NSW and Victoria without the need to quarantine for the first time in almost nine months.
Under changes that came into effect overnight, the states have been reclassified as "very low risk", removing the requirement to self-isolate for 14 days.
It leaves South Australia as the only state still subject to travel restrictions.
SA travellers will no longer require exemptions to enter WA as of Friday, although they will still need to enter self-quarantine.
Queensland will open up to Adelaide from Saturday as long as there are no unlinked cases in coming days.
The decision by WA to proceed with reopening is significant for those separated from loved ones with Christmas around the corner.
But Premier Mark McGowan is warning the hard border could return "in a heartbeat" if circumstances change, revealing the state will retain police officers at border checkpoints at Eucla and Kununurra and at the airport despite the easing of controls.
"If we need to increase the border arrangements to become a hard border again, we can do it basically instantaneously," he said.
Mr McGowan has also ruled out moving to the more targeted hotspot definition used in other states, saying WA's system is simpler and more effective.
The decision by WA to reopen comes after fears of undetected COVID-19 transmission in Sydney were allayed.
No locally acquired cases have been detected after a Sydney quarantine hotel cleaner caught the virus at work last week.
WA introduced 14-day quarantine requirements for interstate travellers at the height of the pandemic's first wave of infections in March.
Mr McGowan went a step further in April, introducing hard border restrictions which denied entry to all but the few who were able to secure exemptions.
The restrictions have been gradually eased as other states got on top of local outbreaks.
A potential threat emerged over the weekend when two German travellers managed to avoid quarantine upon landing in Sydney and instead board a flight to Melbourne.
The duo have since and NSW police have accepted responsibility for the bungle, which forced the entire plane into self-isolation.
"It's certainly not the gold standard," Mr McGowan said, referencing a phrase used by NSW counterpart Gladys Berejiklian to describe her state's handling of the pandemic.
"It obviously shows that COVID is a risk and that's why we continue to have a controlled border to ensure we can protect people into the future."
More than 30,000 people have arrived in WA from interstate since the hard border restrictions were eased on 14 November.
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