He calls it "rocket fuel". Others may be cruder.

Braving freezing winds to survey a paddock filled with lambs grazing on young green oats, Mr Kelly said it could now support four times as many sheep than before it was fertilised a few months ago with biosolids. Biosolids are treated and sanitised human waste with 20 per cent of the water removed.

"Without doubt, these sheep are 20 per cent larger, better grown, mature quicker and have more wool than if they hadn't had access to this paddock," said Mr Kelly. His family, including father Cliff and brother Andrew, run 10,000 sheep on the property Ferndale at Ferndale, about 30 minutes drive from Bathurst.

The roots of the oats were also deeper, the grain bigger and the quality better, he said.

Biosolids are rich in phosphorous and nitrogen. They add carbon to the soil and break down more slowly than synthetic fertilisers, conditioning the soil and making it more drought resistant.

More than 20 years ago, most sludge - what remains after sand, grit and water is removed from the waste flushed down Sydney's toilets and sinks - was shipped to sea.

Now nearly every scrap is being turned into 180,000 tonnes of organic fertiliser a year by Sydney Water. These biosolids are being reused as compost and to rehabilitate mines, but about 70 per cent is being used as organic fertiliser on farms in NSW's west.

You would think Sydneysiders would be full of it, but demand for the biosolids exceeds supply, said agronomist Roger Crisp from Australian Native Landscapes, one of three contractors selling biosolids produced by Sydney Water.

The big issue with biosolids was the volume available: "If we go around saying how great it is people will be saying why isn't everyone using it? There's not enough."

Sydney Water's innovative program was great, said NSW Minister for Energy and Utilities Don Harwin after visiting the Cronulla waste water treatment plant on Thursday as a truck left with another load.

"There's nothing better than the idea that the waste of the cities is, in fact, generating prosperity and income for the bush," he said.

Crop yields have improved by 20 to 30 per cent, he said, while Sydney Water was also saving money by generating 21 per cent of its own power [from the clean gas generated during its production]," he said.

The Kellys first used biosolids in 2012 on their worst field. They were looking for a healthier and more cost-effective way to fertilise than synthetic fertilisers.

"It was expensive upfront, but when you get a payout from it five years later, it was cheap in the long run," said Stuart Kelly.

His brother Andrew said some friends are "quite surprised we are spreading biosolids, human waste".

"It has been through a biological process," he said. "The nutrients in the ground are also breaking it down, and then the sheep are eating [pasture fertilised with] it, and breaking it down it even further into protein and wool. It is that far removed, I am quite satisfied eating meat that is grazed on this. We are doing the environment a favour," he said.

Before farmers can apply biosolids, they need an environmental assessment. Councils and neighbours are notified. The soil has to be tested, governing where, when and how they will use it.

It can't be applied near a hill or near water. Farmers may not graze stock on paddocks treated with biosolids for at least 30 days although usually this doesn't occur until later when the crop is ready, and 90 days, for lactating animals. They can't use it for ground-growing crops such as potatoes, spinach or lettuce.

To remove toxins and bacteria, Sydney Water screens and settles the sludge, which is then baked in digesters at high heat for two periods of 20 days or more.

Over the years it has become better at removing water, making the product more concentrated and cheaper to transport. At the Cronulla plant every litre of waste coming in contains about 180 milligrams of solids, and after biosolids are removed less than one milligram of solids goes to waste.

Like most children's fascination with scatology, everyone in the industry puns and jokes although it is tough to find a new angle.

Humour was a distraction from the smell emanating from the newly arrived black fertiliser - equivalent to about .05 per cent of Sydney's output - that had just arrived at Ferndale last week.

Someone said it looks like the truck spreading the solids has left skidmarks on the otherwise golden paddock.

Later that day, a plough worked the biosolids into the soil, a process Mr Kelly describes as pushing s--- uphill.

Environmental engineer Jacqueline Thomas from the University of Sydney said Australian biosolids guidelines - such as the level of heavy metals, its use and its application - were stricter and more detailed than overseas. As well, the Australian population was very healthy so the risks were reduced. She said Australia's use of biosolids was regarded as a model for other countries.

NSW Health says there's no evidence of an outbreak of illness caused by biosolids.