Three nearby fires have merged to form a "mega fire" the size of
the city itself, and the smoke is so bad it's setting off smoke alarms and
sending asthmatics to the hospital.

The fallout from the hazy air ― the worst on record in the city ― was intense.
Schools canceled sports for the day and work sites shut down. Fire alarms rang
out from buildings and train stations across the city. Visibility was so limited
ferry service was canceled and flights were delayed.

https://mobile.abc.net.au/news/2019-12-11/sydney-smoke-from-nsw-bushfires-explained/11781132