Sydney - Two small planes collided in mid-air in Sydney's south-west Thursday, killing two women, both believed to be in their 20s.

The plane carrying the women, a Cessna 152 aircraft owned by the Basair Aviation College, crashed into the backyard of a house in the Sydney suburb of Casula.

The two women killed were a flight instructor and a student pilot.

The other plane, a single-engine Liberty aircraft owned by the Sydney Flight Training Centre, with an 89-year-old male instructor and 25-year-old male student pilot aboard, made an emergency landing at Bankstown Airport. The two men were unhurt.

Police said a pergola attached to the house was destroyed when the plane crashed into it, but fortunately, the owners and their two-week-old baby were out shopping at the time of the incident.

Two neighbouring homes sustained minor damage from plane debris.

Police have begun an investigation into the circumstances surrounding the crash.

"There's nothing worse in aviation. Unfortunately it is a part of aviation. People suffer accidents on the roads regularly - it happens in aeroplanes too," Basair business development manager Darren Ward was quoted as saying by the national news agency, Australian Associated Press.

Ward told reporters that families, friends and work colleagues were "very upset and very, very distressed" about the deaths.

The 89-year-old flight instructor would have undergone rigorous medical and flying proficiency tests every six months to retain his commercial licence, Civil Aviation Safety Authority spokesman Peter Gibson said.

In September, a 20-year-old Indian trainee pilot died when his single-engine aircraft crashed into farmland in Sydney's Luddenham suburb.

In August, a second Indian trainee pilot died after the wing of his plane was clipped in a mid-air collision with another plane and crashed in the heavily-populated Melbourne suburb of Cheltenham. (dpa)