Demand for the underground tours is so great that people who apply for tickets have been known to offer bribes such as chocolates and flowers to try make sure they are not among the thousands turned away each year, tour organisers said on Monday.

"I've had people calling up and offering all kinds of bribes to try and get a place on the tour," said Pascale Hastings, from Sydney's Historic Houses Trust, which runs the sewer tours.

"There's a huge appeal in going underground and being in dark, confined spaces normally off limits."

The Historic Houses Trust, which manages some of Australia's oldest heritage buildings, organises tours of the old Sydney sewers only twice a year, and can take only 180 people through the drains every six months.

But more than 4,000 people regularly apply for tickets to tour the drains, built to provide fresh water for the British settlement in Sydney in about 1790 by convicts sent to Australia.

By the 1820s, the stone drains were being used as sewers. Today, they carry only storm water to Sydney Harbour, running beneath Sydney's tallest buildings and commercial centre.

The drains are some of the oldest surviving remnants of Australia's early European settlement, after the first British settlers and convicts arrived to set up a British colony in Sydney in 1788.