About a quarter (24 per cent) of the 3134 cases detected last week were not interviewed within a day.

That's up from 10 per cent a fortnight earlier when daily case totals were averaging about 215.

About half of all cases last week remain unlinked to known clusters.

Results from pathology have also slowed with nearly one-third of positive cases last week (32 per cent) being notified of their positive status after more than 24 hours, up from 23 per cent two weeks earlier.

The data was released by NSW Health hours before it revealed the state had found a record 681 cases in a single day.

Deputy Chief Medical Officer Marianne Gale said the state had "absolutely" surged its efforts in contact tracing.

"We have redeployed staff and grown that capacity. As and when needed, we'll call on assistance from other states, as has happened to date," she told reporters on Thursday.

Epidemiologist Catherine Bennett said all tracing systems reached a point where the sheer number of cases were overwhelming.

"It's a truism that the quality of your contact tracing reaches a point where sheer case numbers challenges it," Deakin University's Chair in Epidemiology told AAP on Thursday.

"Even if you scale up your response, it still challenges you."

This situation was a different dynamic to Victoria's well-known contact tracing issues during its major 2020 outbreak, Professor Bennett said.

In Victoria, tracers' computer systems weren't up to scratch and a high proportion of cases were in aged care facilities.

"While there is a very big job in trying to manage those outbreaks in aged care, it's not the same as someone who has come in with symptoms who might have been an essential worker, at home and doing other things that you have to follow up," Prof Bennett said.

But the NSW community had done an amazing job of keeping the reproductive rate at 1.3 for a month, she said.

Delta unrestrained has a natural reproduction rate in the realm of five or six new cases per infection.

"That's an extraordinary effort - and that's everyone's effort, not just the health department," she said.

"Melbourne at the moment hasn't got it down to 1.3."

Restrictive measures were working but people just had to hold on and ensure they took no chances, such as having dinner at another home, she said.

"If people are hearing that contact tracing can't keep up, then know that's exactly why lockdown is so important and why that's going to carry more of the weight ... of containing this virus," Prof Bennett said.

"The contact tracing can't work in the same way when you get to these numbers.

"It's still there, it's still important but it's just not going to be quite the same."

Australian Associated Press