The GPT, which mayor Ben Shields said was "approaching a million dollars", can hold 40 tonnes of rubbish. It also has a life expectancy of 80 years.

Installation of the GPT started in July last year and was finished in December.

Optimal Stormwater's Hugh Williamson said installing the trap was one of the hardest construction projects the company had ever encountered.

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"We were up against not only the wettest August in Dubbo in years but this ground was perfect when it was dry, it was the worst soil when it was wet. It turned into yoghurt," Mr Williamson said.

At 11 metres deep, Mr Williamson said the GPT had to be installed under the river level.

The trap will catch 95 per cent of pollution as long as it's bigger than one millimetre.

Dubbo Regional Council director infrastructure Julian Geddes said there were 64 GPTs installed in the local government area. Of those, 46 are in Dubbo and another 18 are in Wellington.

"In total this means we can collect approx 350 tonnes of waste from going into our river system each year. And during wet years, that increases to around 450 tonnes," Mr Geddes said.

Cr Shields said the GPT needed to be large because of the high level of growth in the north-west area of Dubbo.

"There are so many flow-on effects for good river health that we need to make sure we get right. The old days of just putting trash in the river is totally and utterly unacceptable," he said.

"We want to make our river environmentally healthy and also healthier for people to use and enjoy in a recreational sense."

GPTs help to catch stormwater pollution before it enters waterways like the Macquarie River. They act like a filter by catching the rubbish but allowing the water to flow through.

Since the west Dubbo GPT was installed about 32 tonnes of rubbish have already been collected.

Cr Shields said the GPTs were something worth doing because there was "good economics behind having a good environment".

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