The Greater Darwin and Katherine regions will be declared high-risk by Tasmania's public health authorities from 4pm today.
This means that anyone who has spent time in these areas will not be permitted to enter the state unless they are an authorised essential traveller.
Essential travellers will need to quarantine at an appropriate premises for 14 days.
The decision to close the border to the regions follows a decision for Greater Darwin and Katherine residents to enter a three-day lockdown due to a community COVID-19 case.
Anyone who has recently arrived to the state from the Northern Territory is asked to monitor their symptoms and contact the Public Health hotline on 1800 671 738 should even the mildest symptoms develop.
Public Health Services will provide a further update later.