Dogs and cats have been turning up at veterinary clinics and hospitals across Sydney with severe asthmatic attacks, breathing difficulties and nausea.

Bondi Vet Hospital owner Dr Kate Adams says the reaction of pets to the smoke haze is minor in most cases.

But for older pets or "squishy-nosed breeds" of dogs and cats, as well as those with pre-existing medical conditions, the smog can be life-threatening.

Dr Adams said owners should look for signs their pets are struggling, such as breathing difficulties, coughing, vomiting, lethargy, and runny noses and eyes.

"Pets can't tell us, 'Hey, by the way, slightly asthmatic over here', like people can," Dr Adams told AAP.

RSPCA NSW spokeswoman Sharon Andronicos said keeping pets of all species indoors was the safest bet.

"If it is uncomfortable for a human to breathe, then it is uncomfortable for pets too," Ms Andronicos said in a statement.

"I would suggest not doing strenuous activities or big walks until the smoke clears."

Dr Adams said owners should also turn on the air-conditioner to help filter the air and keep their furry friends cool.

She said keeping energetic, bored pets inside isn't ideal, but the unprecedented level of smoke makes it a necessity.

Sydney's east and southwest were rated as "hazardous" for air pollution on Friday, while the northwest had "fair" conditions.

The NSW environment department said this season's bushfire emergency has caused "some of the highest air pollution ever seen in NSW", with hazy conditions "the longest and the most widespread in our records".

Australian Associated Press