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A huge landslip at a model railway construction site in Waitakere bushland has fuelled Swanson residents' concerns about the project.

The slip, which crashed down into the Swanson Stream during heavy rain, has formed a dam, causing water to rise by four metres and spill over on to the Redwood Park Golf Course.

Land owner Ross Britten, a former Waitakere City Councillor, said the slip began on the council stream-bank reserve and undermined land below the railway line and botanical garden which he is building.

Yesterday, a scene of devastation greeted officers from the Auckland Regional Council and city council.

A scar of yellow clay stretched 20m up the steep slope to where Mr Britten had formed a road.

Uprooted mature native trees and ferns were among debris which formed a dam 15m long.

Swanson Stream is a key part of the city's $55 million Project Twin Streams to prevent flooding east of the Waitakere Ranges. Volunteers are planting thousands of shrubs.

Swanson Residents and Ratepayers Association president Meredith Youngson said residents had long raised concerns about the stability of the steep slope where Mr Britten was forming the railway line and embankments using old tyres.

"Each time we have been assured by Mr Britten that the appropriate geotechnical work has been done.

"Now, we are not sure."

Residents were concerned about how truckloads of rubble and tyres were being compacted on Mr Britten's project.

Mrs Youngson said she expected the city council to take a close look into the landslip and ensure that the rest of the site did not slip.

"It could be really disastrous as well as an eyesore," she said.

But Mr Britten, who only last September was granted a resource consent by the city council to finish the project, said the landslip was "an act of God ... the place has been correctly engineered".

He said the slip was caused by the stream bank becoming saturated and he was working with both councils to install drains to divert water from the slip area.

The slip was a setback for his dream of 23 years to create a 2km scale model railway track, with tunnels and landscaped gardens, but "we are not abandoning the project".

A consultant to the regional council, Graeme Ridley, said the ARC and city council were concentrating on getting the stream flowing back in its natural course and preventing any further slips.