The Inge Kosan, an LPG tanker, was docked in Port Botany for a day from March 31 after arriving from the Papua New Guinea capital of Port Moresby.

It then sailed for Vanuatu on April 1.

"Health authorities in Vanuatu have confirmed that 12 out of 13 crew members from the ship, including (a) deceased crew member, tested positive for COVID following testing in Port Vila," NSW Health said in a statement on Friday.

Some 15 wharfies boarded the vessel when it was at Port Botany more than three weeks ago and all have since tested negative.

NSW Health said none of the crew of the Inge Kosan left the ship when it was in Sydney and the wharfies who boarded it were wearing protective gear.

NSW thus recorded no new locally acquired COVID-19 cases in the 24 hours to 8pm on Friday, and reported one case in hotel quarantine.

The Maritime Union of Australia nevertheless on Friday demanded National Cabinet improve testing and biosecurity measures for arriving vessels.

The MUA's demands include the introduction of rapid testing for arriving seafarers and the declaration of Australian maritime workers as "key workers", making them eligible for vaccination sooner.

Meanwhile, people who arrive in NSW from Western Australia from Saturday will be obliged to comply with the same lockdown provisions imposed on the Perth and Peel regions for at least the next three days.

The WA government has imposed a snap lockdown on those regions after two COVID-positive people spent time in the community while infectious.

NSW authorities encouraged anyone who has been in WA since April 17 to check the WA list of sites of concern and self-isolate if required.

NSW Health also said on Saturday that COVID-19 fragments had been located in sewage systems at Allambie Heights, on Sydney's northern beaches, and Merimbula on the state's south coast.

However these fragments may have come from recently recovered COVID-19 patients.

Australian Associated Press