SYDNEY: Air quality in Sydney plummeted on Tuesday to levels among the world's worst as smoke from bushfires in the north blanketed the harbour city, taking large swathes into index ranges on par with New Delhi, one of the world's most crowded capitals.

Although smog is rare in Sydney, better known for its beaches and blue skies, grey clouds hovered over the iconic Opera House and Harbour Bridge, with smoke visible in the air.

The air quality index hit 161 in areas just north of the city's downtown after 5 p.m. (0600 GMT), reaching a level at which people are advised to avoid prolonged outdoor exercise.

But there could be relief in sight, with the Rural Fire Service (RFS) saying southerly winds should begin to dispel the smog by evening.

The smoke had drifted hundreds of kilometers south from fires burning across roughly 136,000 hectares (336,000 acres), an area almost the size of Greater London, the RFS said.

Authorities have warned of a high-risk bush fire season in Australia this summer after two quiet seasons, compared with the 2019-2020 "Black Summer" fires that destroyed an area the size of Turkey and killed 33 people.

Energy company Santos halted work on a gas project northwest of Sydney as a huge bushfire raged out of control in the Pilliga forest south of Narrabri, a town that is about 420 km (261 miles) away.

"The fires in the Pilliga are of enormous concern," a company spokesperson said in a statement. "While there is no risk to, or because of, our facilities, we have shut in our operations for the time being."

In the Indian capital, where index levels range between 74 and 225, scientists recently planned to turn to cloud seeding to ameliorate matters.