The Northern Territory has declared South Australia a coronavirus hotspot with the rapid spread of a COVID-19 cluster in Adelaide prompting authorities to go "hard and wide".
Chief Minister Michael Gunner says anyone travelling to the Territory from SA will now be forced into supervised quarantine for 14 days.
He says anyone arriving on Monday would be given the option to return to SA.
"The security committee has just met to review the alarming developments in South Australia overnight," he said.
"All of the information that we are getting right now concerns us and there is still so much we don't know about this outbreak.
"That is the critical point here. It is what we don't know that worries us the most."
The SA cluster was sparked by a worker at a quarantine hotel becoming infected and spreading it to other family members.
Of the 17 cases detected so far, 15 are thought to involve members of the one extended family.
Chief Health Officer Hugh Heggie said the hotspot declaration had been made to protect the health of all Territorians.
"The outbreak in South Australia has grown quickly and the decisions made in the coming days to contain it will be critical," he said.
"This is why I have made the decision to go hard and wide and declare all of South Australia as a hotspot.
"I will continue to review and assess the situation and respond accordingly."
Mr Gunner said it was a case of presuming the worst might eventuate in SA and preparing accordingly.
He said anyone arriving in the NT on Monday or Tuesday would not be required to pay the $2500 quarantine fee given the short notice of the changes.
"For those who are going to be caught up in this, I am sorry," he said.
"No-one wants this, but it has to be done."
The NT's crackdown on SA came as Mr Gunner also announced that people from greater Melbourne would be able to enter the Territory without the need to quarantine from November 30.
It came as Victoria recorded its 17th straight day of no new COVID-19 cases.
"Like we have in the past, we are announcing a date well in advance to give us the time we need to be extra sure and extra safe.
"Waiting another 14 days gives us two full replication cycles of data to be absolutely confident.
"This is happening much sooner than many of us expected. If you think back to a few months ago, very few were predicting that Melbourne would become so safe so quickly."