NSW records 31 new locally acquired COVID-19 cases

"We are anticipating there could be an increase in numbers over the next few days, I then hopefully early next week we should see the impact of the lockdown really turning and having a positive impact."

Close contacts 'Stay at home'

Ms Berejiklian is emphasising that if you're a close contact, you shouldn't be leaving your house.

"That includes for vaccination, that includes for medical attention. If you are a close contact, please ring Health and Health will meet all of your needs.

"If you need medical attention, ring Health. If you need other supplies you cannot get, please ring Health. If you are a close contact, you should not be leaving the house. If you are deemed a casual contact, please follow the health advice."

Delta COVID-19 variant is more infectious

But there's 'promising' data on vaccines and deadliness

Delta COVID-19 variant is, has different symptoms, and has been described as everything from a "Tricky bugger" to a "Guerilla fighter" by state politicians.

As the Delta variant spreads, experts are learning more about how it works.

How contagious is the Delta variant?

It's far more infectious than the virus we were dealing with last year.

In the United Kingdom the Delta variant only emerged in April, but it has quickly taken over as the dominant form of the virus there.

Experts estimate it could be about 50 per cent more transmissible than the Alpha variant, which was already about 50 per cent more transmissible than the original virus found in Wuhan.

"Compared to the original strain, the Delta strain is two times more contagious," said Professor Stuart Turville, a virologist from the Kirby Institute.

The "Secondary attack rate", or the number of people who are likely to be infected if they come into contact with an infected person, is also higher.

If 100 people come into contact with someone with the Delta strain, 12 of those people are likely to be infected.

What are the Delta variant symptoms?

Previously fever, persistent cough and loss of taste or smell were the top symptoms to watch out for.

But with Delta, a headache is the most common symptom, followed by a sore throat, runny nose and fever.