Anyone who has been in Victoria since July 8 won't be allowed to enter Tasmania unless they receive permission from the deputy state controller as an essential traveller.

The directive will be in force from midnight on Thursday and reviewed daily.

"We're responding to where the risk is. Victoria are concerned about the risk and the need for their contact tracers to catch up," Tasmania's Deputy Director of Public Health, Scott McKeown, told reporters.

"We need to see what risks and cases emerge over coming days."

Victoria is entering a snap five-day lockdown from 11.59pm on Thursday after the state recorded an additional two COVID-19 cases, bringing the outbreak to 18.

Tasmania, which is free of the virus, is already closed to Sydney and a swathe of NSW local government areas including Parramatta, Penrith and Wollongong.

Dr McKeown said NSW areas outside of Sydney remained low risk based on current assessments.

He says health authorities will obtain genomic sequencing in relation to the case of a health worker who tested positive in the UK after leaving Tasmania with a negative test.

Tasmania ordered people who visited venues at the same time as the worker to get tested as a precaution.

Dr McKeown said there was no evidence the worker caught the virus or was infectious in Tasmania, adding wastewater test results would be returned on Friday evening.

Australian Associated Press