A Qantas flight bound for Melbourne was forced to return to Sydney after a suspect vapour trail was seen coming from the plane's undercarriage.

Domestic flight QF411 departed for Melbourne with 230 passengers at 7.30am (AEST) but returned to Sydney within minutes, a Qantas spokesman said on April 10.

Air traffic controllers ordered the plane back when they saw the vapour streaming from the Boeing 767.

Emergency crews were on standby but the plane landed safely and made its way to the domestic terminal, where passengers disembarked.

None of the passengers seemed noticeably shaken by the experience, the spokesman said.

A preliminary inspection by Qantas revealed the problem was caused by a loose hose clamp from which water had leaked from the plane.

Qantas has provided a replacement aircraft, while some passengers have already boarded other flights to Melbourne.

Inspection crews will continue their investigation.