The Hong Kong government had been told to revamp air pollution rules, say officials.
The order from the city's ombudsman comes several months after Friends of the Earth Hong Kong asked the Chinese territory's leaders for an explanation of its outdated air quality objectives, largely unchanged since the late 1980s.
The environmental group filed a formal complaint last year over the city's pollution rules.
The watchdog's decision is non-binding but Lau said the order was "a little bit of a victory" since it could not force government action on the issue, and stopped short of accusing officials of "maladministration."
A spokesman for the ombudsman could not be reached Sunday, but the watchdog previously declined to comment on its probe, citing privacy laws.
The city's Environmental Protection Department was quoted by the Sunday Morning Post as saying the ruling was "welcome" and it was "working closely with the concerned bureaux and departments and other stakeholders with a view to drawing up a practicable timetable."
Hong Kong's roadside air pollution hit record levels last year as air quality continues to be a big health issue in the city of seven million, with critics warning that it would drive away talented professionals.
A survey last year by public policy think tank Civic Exchange found one-quarter of residents would like to leave Hong Kong to escape its pollution.
Emissions from factories in southern China, which seep over Hong Kong's border, combined with local emissions from power plants and transport, generate an almost daily thick blanket of haze over the teeming metropolis.