Sydney residents have breathed through 38 days of "very poor" air quality since the start of November, including 28 days considered to be "hazardous".

Amid calls from the NSW Opposition for Premier Gladys Berejiklian to intervene with a "specific plan" to deal with poor air quality across the state, Sydney was again blanketed by smoke on Thursday.

Sydney again disappeared under a blanket of smoke on Thursday amid worsening bushfires across NSW. Credit:Dominic Lorrimer

The state government declared a state of emergency as the thick smoke haze shrouded the city, temperatures soared past 40 degrees, and worsening fires ripped through bushland across NSW.

More than 100 additional paramedics will be on duty in coming days as residents - particularly the elderly, children, and those with respiratory problems - are urged to be vigilant about their health.