COVID-19: New South Wales records 136 new cases, the highest in a single day since outbreak began

New South Wales has recorded 136 new cases of COVID-19  - the highest number in a single day since the outbreak began - with state Premier Gladys Berejiklian calling the upsurge a "national emergency".

Of the 136 new cases - recorded in the 24 hours to 8pm on Thursday (local time) - 53 were infectious while in the community, local health officials said on Friday. Seventy-seven are linked to a known case or cluster, with 65 classified as household contacts and 12 as close contacts. How 59 of the cases became infected is currently unclear and under investigation.

Fifty-three cases were in isolation throughout their infectious period and 17 were in isolation for part of it, health officials said. The isolation status of 13 cases remains under investigation.

The stricken state is grappling to control its escalating outbreak of the virus, with a stay-at-home order currently in place until Friday, July 30. However, it's understood those restrictions will be extended.

Measures were further tightened in several local government areas in south-west Sydney on Friday, with additional restrictions imposed in Cumberland and Blacktown. In line with the restrictions currently implemented in Fairfield, Liverpool and Canterbury-Bankstown, residents of both Cumberland and Blacktown will no longer be allowed to leave their communities unless they are an authorised worker.

"There is no doubt that the numbers are not going in the direction we were hoping they would at this stage," Premier Berejiklian said during a press conference on Friday.

Chief Health Officer Dr Kerry Chant also said an 89-year-old man had died overnight, but no further information would be released until the family had been notified.

Additionally, five recent returnees - or overseas-acquired cases - also tested positive in the 24 hours to 8pm on Thursday.

NSW has recorded 7303 cases of COVID-19 to date, with 1782 locally acquired cases reported since June 16, 2021 - when the first case in the Bondi cluster was reported.

Currently 137 people with COVID-19 have been admitted to hospital, with 32 people in intensive care - 14 of whom require ventilation. The state's death toll stands at 61.

In the 24 hours to 8pm (local time), 86,620 tests were recorded - the highest number to date - compared with the previous day's total of 85,185. It marks the third day in a row that the state has seen a new testing record, with health officials thanking the community for coming forward in such high numbers.

NSW Health administered 25,610 vaccines in the 24 hours to 8pm - the highest number in a single day.

The total number of vaccines administered in NSW now stands at 3,293,898, with 1,284,474 doses administered by NSW Health to 8pm and 2,009,424 administered by the GP network and other providers to 11:59pm on Wednesday.

Earlier on Friday morning, infectious disease expert, Professor Robert Booy, said NSW "is on the precipice" of losing control of its sprawling outbreak if transmission continues in the community.

"It's so close. It is still possible to get control over the next week or so," Professor Booy told Today.

"With so many in the community infectious, different chains of transmission, unrecognised chains of transmission, it's a real challenge."

Queensland has once again closed its border with NSW in a bid to keep the highly transmissible Delta strain out of the northern state, which has recorded three new cases in the last 24 hours.

Fragments detected in Byron Bay's sewage systems

Hundreds of police are now manning checkpoints along the border after traces of the virus were detected in sewage treatment plants at West Lindfield and Byron Bay, where there are no known cases.

It is the first detection this year at the Byron Bay sewage treatment plant, which serves about 19,000 people in Byron Bay, Wategos, Suffolk Park, Sunrise, and Broken Head, local officials said - raising fresh fears that COVID-19 may have spread outside of Sydney again.

It is also the first detection of fragments at the West Lindfield sewage treatment plant, which serves almost 118,000 people.

"There are no known cases in this area, which is of great concern," the state's health department said on Friday.

Authorities have asked everyone in the Byron Bay area to be on high alert for any symptoms, and if they appear, "to immediately come forward for testing and isolate until a negative result is received".