SYDNEY, July 1 (Reuters) - Strong winds and heavy rain led airlines to cancel many domestic flights from Sydney on Tuesday as Australia's weather bureau warned an intense low-pressure weather system offshore could result in flash flooding in parts of the country's southeast.

Qantas Airways QAN.AX cancelled at least 11 domestic flights operating from Sydney and Virgin Australia VGN.AX cancelled 12, the Sydney Airport website showed. No international flights have been affected.

"Some services on Virgin Australia's network have been impacted by adverse weather in Sydney and Newcastle today," a Virgin Australia spokesperson said by email.

Australia's Bureau of Meteorology said wind gusts of up to 125 kph (78 mph) may hit Sydney's coastal regions later on Tuesday, while rainfall of up to 120 mm (4.7 inches), roughly the mean for July, could be dumped in some places in six hours.

"The system is quite dynamic and quite vigorous, and we are expecting conditions to escalate through the afternoon," weather bureau forecaster Gabrielle Woodhouse told reporters.

"This means that we have a greater flash-flood risk with potential for some fairly heavy falls to develop over a short period of time."

The weather system is expected to shift offshore into the Tasman Sea on Wednesday and ease by Thursday, Woodhouse said.

More than 1,000 volunteers have been mobilised in New South Wales to respond to the weather event, the state's emergency services said.

(Reporting by Renju Jose in Sydney; Editing by Jamie Freed)

((renju.jose@thomsonreuters.com;))