The fish, called koi, have reproduced to such numbers and grown to such size at the Shoyoen Sister City Garden that they’ve disrupted the banks of their lake.

Dubbo City Council parks and landcare director Ken Rogers told councillors the bank was not yet in danger of collapse, but he did not rule out a cull.

The colourful koi delight visitors to the tranquil gardens and readily accept a feed in addition to the meals they find themselves among the rocks.

Their environment obviously suits them because there’s been a population explosion.

Dubbo City Council horticultural services manager Ian McAlister said council would have to tally the fish before it could decide what to do.

“There were 15 koi to start with in 2002, I couldn’t tell you the numbers now, but there’s been a hell of an increase,” he said.

The fish burrowed under the rocks so that those on the bank had fallen a small distance, he said.

The base of the lake was pipe clay but the kois’ activities in it - they are bottom feeders - make the water cloudy, he said.

“We weren’t aware it was going to be a major issue, but it’s certainly a manageable one,” he said.

He had no definite answer for how to solve the problem, but said he did not want to destroy the koi.

Council will drop the water level at the end of September, count the fish and fix the damage.

Then it is decision time.

“There is the potential to sell some of them,” he said.

“We may offer them to other Japanese gardens.”

The kois’ destruction was discussed at the August meeting of the council.

Cr Richard Mutton asked if the carp were eating the pond.

“They are undermining the bank of the lake at the Japanese garden,” Mr Rogers said. “It won’t collapse at the moment, but we’re looking at options.

“There may be a cull - we’re looking at the most cost-effective way,” he said.

The garden was a living gift from the people of Minokamo, Dubbo’s Japanese sister city.

The Friends of Shoyoen and Elizabeth Park are an enthusiastic group who guide visitors and assist with maintaining the gardens, meeting there on Wednesday mornings.

Mr McAlister said anyone interested in becoming a friend could contact council.