The NSW government on Thursday declared the second week-long state of emergency of the bushfire season amid dangerous fire conditions and temperatures soaring above 40C statewide.

Hours after Premier Gladys Berejiklian made the declaration, Australian Council of Trade Unions secretary Sally McManus said outdoor workers - about one-in-four of the country's workforce - needed additional protection.

She said the ACTU would like to see federal work, health and safety laws updated to deal with hazards like heat and smoke exposure.

Australia currently has non-binding guidelines setting out how employers manage heat, with regulations established on an industry-by-industry basis. Ms McManus said non-critical outdoor work in Australia should be barred above temperatures of 38C.

"Extreme heat kills, it's killed 13 workers in the recent history ... it puts a lot of pressure on the body," Ms McManus said.

"We need to also update our laws to deal with the fact that climate change is here and so that workers right across the country have the protection of laws when it comes to issues of extreme heat and also about smoke pollution."

Air quality in Sydney was so thick at one point last week that it was deemed 11 times worse than typically hazardous levels.

"In some ways, we won't even know the effect of this particular climate emergency and smoke emergency we've had in Sydney for years to come," Ms McManus said.

Some industries which are unionised, such as construction, already have enforceable heat regulations, Ms McManus said.

The union said outdoor workers should also be taking regular breaks as temperatures reach 30C.

Australian Associated Press