Prime Minister Scott Morrison has rejected paying volunteer firefighters battling an unprecedented number of bushfires up and down the eastern seaboard, claiming that they "want to be out there".

The prime minister was speaking at a press conference to spruik his revamped religious freedom legislation.

As he addressed reporters on Tuesday afternoon, about 2700 firefighters were tackling 85 fires, 42 of which were uncontrolled. And that was just NSW.

Working well

"I was speaking with the (NSW RFS) commissioner on the weekend out at Wilberforce - where we have the megafire out in the northwest at the moment - we were talking through the crew rotations and the fact is that these crews, yes they're tired, but they also want to be out there defending their communities," he told reporters.

Chevron Right Icon 'They also want to be out there.'

"We do all we can to rotate the shifts to give them those breaks but equally, in many cases, you've got to hold them back to make sure they get that rest. I thank them all for what they're doing."

Morrison dismissed the idea of paying volunteer firefighters.

Your cookie settings are preventing this third party content from displaying. If you’d like to view this content, please adjust your Cookie Settings . To find out more about how we use cookies, please see our Cookie Guide.

"We are constantly looking at ways we can better facilitate the volunteer effort but to professionalise that, at that scale, is not a matter that has previously been accepted and is not a matter that's currently under consideration by the government," he said.

"I can reassure everyone the nationally co-ordinated effort and the specific state efforts leading the response in each of their jurisdictions has been incredibly professionally deployed."

Morrison said the defence force was also helping state fire authorities and thanked US, Canadian and New Zealand firefighters helping in Australia.

Prime Minister Scott Morrison says there is no need to pay volunteer firefighters. Credit: Jeremy Piper/ Joel Carrett/ Paul Braven / AAP

Welfare fears

But former Fire and Rescue NSW commissioner Greg Mullins voiced his fears for the welfare of volunteers on Tuesday afternoon while himself on the way to fight a blaze at Mangrove Mountain on the state's Central Coast.

Mullins said crews were working 12-hour shifts and with many travelling two hours each way to reach fire grounds, meaning they faced 16 hour days.

Chevron Right Icon 'Everyone is getting pretty worn out ... that's where all the bad stuff happens.'

"Everyone is getting pretty worn out and, in my experience, that's where all the bad stuff happens," he told AAP.

"The troops, if they're worn out, you get to a stage where it just overwhelms everyone, so that's a worry."

He said as the bushfire season continued and volunteers kept requesting time off work their employers would start saying: "Hey we've got a business to run."

There were 83 fires burning across NSW on Tuesday evening, 40 of them uncontrolled and 12 at watch and act level. Credit: AAP/GETTY

More on 7NEWS.com.au

Agencies such as Fire and Rescue NSW and the National Parks and Wildlife Service had a limited pool of people to call on and "they'll all be feeling the same thing", the ex-fire chief said.

Morrison's comments came as Sydney was covered with smoke from a ring of fires around the city.

Smoke alarms

Ferries were cancelled because of poor visibility and office blocks were evacuated as smoke alarms triggered throughout the day.

Authorities say heatwave conditions this week combined with "grotty" smoke pollution could stress vulnerable people in the coming days.

A smoke haze from bushfires in NSW blankets Sydney Harbour. Credit: AAP

"It's going to be putting a lot of stress on vulnerable people, particularly elderly people who have existing heart and lung conditions," NSW Health environmental health director Dr Richard Broome told reporters.

RFS headquarters at Sydney Olympic Park was briefly evacuated on Tuesday with heavy smoke setting off alarms.

Nine regions were under total fire bans on Tuesday including Greater Sydney, the Greater Hunter and the Illawarra-Shoalhaven.