A huge colony of fruit bats that live in Sydney's Royal Botanic Gardens are such a pest that desperate officials have taken to blaring industrial noise and banging sounds to scare them off.

And the ploy appears to have worked.

A week ago some 5,000 of the large mammals could be seen hanging in trees throughout the park, a popular haunt for both locals and tourists on the harbour near the city's famous Opera House.

But now just 10 are left, said the executive director of the Royal Botanic Gardens Brett Summerell.

"The majority of bats have been evicted from the gardens," he said Tuesday.

"We've been playing a range of industrial type noises -- basically annoying sounds for about 45 minutes in the mornings and 35 minutes in the evening."

Over the past 20 years, the bats, also known as flying foxes, have caused extensive damage in the gardens, with more than 28 trees and 30 palms already lost and several hundred more trees and plants damaged, 60 critically.

Summerell said 100 bats had been fitted with satellite collars to track their movement and they were dispersing around the Sydney region, up the coast, and even as far as Queensland state, some 700 kilometres (434 miles) away.