Seven days of smoke and ash from the New South Wales bushfire emergency have taken a toll on Port Macquarie, which on Friday experienced the world's lowest air quality rating.

Key points: Port Macquarie's air pollution rating is 1,739 — anything over 200 is hazardous

Port Macquarie's air pollution rating is 1,739 — anything over 200 is hazardous The air quality in the region is so bad that children are being sent home from schools

The air quality in the region is so bad that children are being sent home from schools Many beaches in the area were blanketed by ash

The State Government told parents to collect their children from schools in the Mid North Coast city as conditions deteriorated.

At 4:00pm, the Environment Department gave Port Macquarie an Air Quality Index (AQI) of 1,739, warning that all residents should stay inside as much as possible.

Port Macquarie's deteriorating air quality meant it was rated significantly worse than pollution hotspots like New Delhi and Beijing.

Air Quality Index (AQI) on November 15: Location AQI Port Macquarie, NSW 1,739 Gurugram, New Delhi 817 Aguascalientes, Mexico 769 Lahore, Pakistan 549 Beijing, China 192 Sydney, NSW 54

At Nambucca, about 100 kilometres north of Port Macquarie, striking images of beaches covered in bushfire ash showed the toll the blazes had taken on the environment.

Ash washed up on Nambucca Beach following the bushfires November 15, 2019 ( Supplied: Renee Tonkin )

Senior lifeguard James Turnham saw the ash washing up on the beaches.

"The beaches have a dusting of ash and the high-tide lines have black ash and burnt leaves, but the water is mostly clear," he said.

An Environment Department spokesperson said smoke from bushfires and increased dust because of the drought were among the factors leading to the poor air quality.

Air pollution becomes "hazardous" when the rating is over 200. The AQI is a scale used by governments and organisations to inform the public how polluted their local area is.

Earlier this week, NSW Premier Gladys Berejiklian declared a "state of emergency" due to the bushfires, which have so far killed four people and destroyed more than 300 homes.

Jason Koenig, who lives at Kew near Port Macquarie, and his wife were asked to pick up their three young children from school due to the air quality.

"The schools are doing their best to minimise the smoke but it is still very dark and hazy," he said.

"It's difficult to breathe, so a lot of the kids would be having breathing difficulties."

Port Macquarie's Lighthouse Beach was barely recognisable during the height of the bushfire emergency. ( Supplied: Laura Begbie )

Further north there were widespread delays and cancellations at Coffs Harbour airport.

There have been increases in the number of hospital presentations for asthma and breathing problems in areas worst affected by the NSW bushfire emergency, prompting a renewed call for people to take precautions when there was smoke in the air.

Emergency Department data shows that hospitals on the Mid North Coast, where fires were at their worst, have had 68 presentations to hospital emergency departments for asthma or breathing problems over the past week — almost double the usual number.