Sydneysiders are in for a hazy weekend as smoke enters the Sydney basin from backburning efforts to contain large blazes on the southern fringe.

The Office of the Environment and Heritage has warned air quality in the basin is likely to be affected by backburning at Holsworthy and from separate hazard-reduction burning at Ironbark Ridge, on the city's north-western edge.

Backburning on Friday to contain the Mackel Airfield fire will likely push smoke towards southern Sydney and the Illawarra over the weekend. Credit:NSW Rural Fire Service

A blaze in bushland at the Holsworthy Barracks site would likely require backburning of as much as 4500 hectares to contain the fire, Ben Shepherd, a senior spokesman for the Rural Fire Service (RFS), said on Friday.

Ground crews were limited in their ability to directly tackle the fire because of the risk of unexploded ordnance from training exercises in the past, Inspector Shepherd said.