Authorities underestimated the significance of a superspreader birthday party that seeded COVID-19 in western Sydney, an inquiry has been told.

The NSW government was not aware of the extent of the COVID-19 outbreak stemming from a "superspreader" party in western Sydney when it waited ten days to lock down parts of the state, a parliamentary inquiry has been told.

Chief Health Officer Kerry Chant and Health Minister Brad Hazzard were quizzed about their handling of the recent coronavirus outbreak at an inquiry on Tuesday.

"It was the seeding event in western Sydney ... West Hoxton that wasn't recognised at the time," Dr Chant told the inquiry.

"It was thought that cluster had been identified very early but there were issues around containment of that, that weren't appreciated," Dr Chant said.

"Obviously with the benefit of hindsight, there are different decisions that can be made."

Just over 30 people attended the West Hoxton birthday party dubbed a "superspreader" event in late June, before the eastern suburbs were subsequently locked down days later.

Greens MP David Shoebridge asked why this was the case.

"The initial intelligence was the West Hoxton party was effectively controlled, everyone immediately contacted within the timeframe," Dr Chant said.

"There were subsequent emerging issues associated with that and I'd be happy to reflect on the thinking at that particular time."

Greens MP Cate Faehrmann asked why Premier Gladys Berejiklian had announced the state could ease restrictions after 50 per cent of people were vaccinated, contrary to advice from the Doherty Institute.

"The Doherty Institute modelling shows 50 per cent, 60 per cent, or even 70 per cent levels of vaccine coverage would not allow for relaxed restrictions unless we're prepared for a massive uptake in hospital admissions and death," Ms Faehrmann said.

"Professor Doherty himself said if we open up at 50 per cent that would be insane."

While Ms Berejiklian's government hopes to hit six million jabs by month's end, Dr Chant advised the state would still need some level of restrictions as it increased its vaccination rate to 70 per cent and above.

"I have not provided specific advice about any restrictions I would be prepared to ease. It is too premature," she said.

"I am actually optimistic that at the end of August we are going to have a higher than 50 per cent first dose coverage."

Mr Hazzard said the premier was trying to give the community a sense of hope and drive up vaccination rates, and would need to know more before easing public health orders.

The health minister disputed suggestions the government had not listened to the health advice regarding the "moving feast" that was the COVID-19 Delta strain outbreak.

Sydney's eastern suburbs response to the initial outbreak had been done "very well and very quickly," he said.

He repeatedly reminded the committee the pair had volunteered their time and were not obliged to answer any questions.

"It would be the first time in history that in the middle of a war, a parliamentary committee called an inquiry to ask us how did you make your decisions," he said.

Mr Shoebridge said the NSW government was less transparent than Victoria after Premier Daniel Andrews was available "for the better part of a day" at one of that state's inquiries.