It brings the territory's latest cluster to 11 after Ms McCarthy's 30-year-old sister and a 43-year-old vaccinated Aboriginal man were reported as infected on Monday.
The new cases are household contacts of the pair, who live in Katherine, about 320km south of Darwin, Chief Minister Michael Gunner says.
"This is a serious escalation of the COVID-19 situation in the NT," he said on Tuesday.
They include a 71-year-old man, two five-year-old twin girls and a 65-year-old woman, who has been admitted to Royal Darwin Hospital.
Four women aged 62, 40, 38 and 22 have also been diagnosed with the virus, along with a 16-year-old girl.
All have been moved or are in the process of being moved to the Centre for National Resilience, with the exception of the woman moved to RDH.
It is not known if the group were vaccinated.
"I know this is a serious development. We are doing all to make sure you are safe," Mr Gunner said, addressing the people of Katherine.
Contact tracers have identified 161 close contacts.
However, no new cases have been diagnosed in the remote Aboriginal community of Robinson River, where Ms McCarthy's sister had travelled and was diagnosed.
Ms McCarthy's sister's case is the first COVID-19 infection reported in a remote NT Aboriginal community.
"I'd like to say a personal message and that is that to my sister, who is the source of the COVID infection in Robinson River, our thoughts and prayers are with her and our family," Ms McCarthy told reporters.
Health teams have been sent to Robinson River, Katherine and surrounding communities for a testing and vaccine blitz.
Greater Katherine and Robinson River, about 1000km from Darwin, were plunged into a lockdown on Monday evening.
That has now been extended in Katherine until Monday with a territory-wide order to wear face masks in most public areas.
Mr Gunner warned the virus could have "seeded" and moved to other communities in the NT.
He said more lockdowns and lockout were possible.
About 350 people live in the community, with 77 per cent fully vaccinated and 87 per cent have had their first dose, according to NT Health.