NSW on high alert as mutant cases double

Authorities are on edge this morning as cases of the highly-transmissible mutant coronavirus strain double in NSW within a week.

Despite recording zero locally transmitted cases yesterday, NSW Health's Dr Jeremy McAnulty said 13 cases of the so-called "UK" strain had been confirmed in returning travellers in Sydney.

It's a huge jump in numbers since January 8, when only six had returned a positive result in Sydney's Special Health Accommodation.