Australia is battling a second wave in its south-east, with about 8000 active cases in Melbourne and smaller clusters in Sydney.
But elsewhere around the country, the virus has effectively been eliminated.
Prime Minister Scott Morrison has said interstate travel restrictions will likely persist until Christmas.
Last week, Queensland re-closed its borders to New South Wales - which includes Sydney - and the Australian Capital Territory.
Officials in the sparsely-populated NT, which hasn't had an infection since May, said an "18-month window" barring visitors from virus hotspots was just a "conservative" forecast.
People from those hotspots can still enter the territory but must complete a mandatory 14-day hotel quarantine at their own cost, which has been an effective deterrent.
Chief Minister Michael Gunner said he was determined to prevent spread from Victoria - of which Melbourne is capital - and New South Wales.
Borders have been open to other states' residents since 17 July.
"We have got an indefinite ban on Victoria, and Sydney keeps bubbling away to a point so I can't give you a date where that would ever lift," Gunner said.