There were no AQHI readings for that station throughout the day until 6.30pm, when it recorded a 7 on the index.

But at the same time, another 10+ reading was recorded at the roadside monitoring station in Causeway Bay. The department confirmed the reading.

The 6.30pm reading marked the first time the new system hit its highest level.

The department issued its most severe health advice, urging children, the elderly and people with heart or respiratory illnesses to avoid outdoor activities, especially in areas of heavy traffic.

The 12 general monitoring stations recorded AQHI levels of 6 (moderate) to 9 (very high). The other two roadside stations, in Central and Mong Kok, recorded an 8 and a 9, respectively.

The index analyses three-hour average concentrations of ozone, nitrogen dioxide, sulphur dioxide and tiny particles (PM2.5 and PM10), using a scale from 1 to 10, and 10+. It is divided into five health-risk categories - low, moderate, high and very high.

Clean Air Network chief executive Kwong Sum-yin said she was not surprised at the high readings as December was usually a more polluted month.

The pollution was caused by bad roadside air quality mixed with secondary background pollution being blown in from the Pearl River Delta, she said. Seasonal northeast monsoon winds tend to affect southern China during the winter.

Kwong urged the department to carry out more frequent maintenance at monitoring stations to avoid technical failures.

"Citizens may become more wary of the department's readings if there are so many problems," she said, adding that a trial run of the index last week saw at least six malfunctions at various stations.

Video: How to deal with Hong Kong's smog