The current facility can service 40,000 but to ensure the plant keeps up with the demand of the city's growing population, the $21.5 million upgrade will be able to treat the waste of an additional 15,000 people.

Dubbo City Council's 2014-15 operational plant and budget forecast a population of 51,000 at Dubbo by 2036.

Updates include the construction of two bioreactors, measuring 22 metres, by 72 metres, by six metres deep, that will utilise micro-organisms to treat the waste material.

NSW Public Works project manager David Watson said 3000 cubic metres of concrete had already been poured for the floors of the tanks.

Mr Watson said, physically, the project was halfway through and construction should be completed by the end of June, but contract payments were only about one-quarter finished.

He said the updated facility was expected to be fully operational about September.

The project is one of the largest in terms of single capital expenditure in the city, alongside the $23 million upgrade to the water treatment plant in 2006 and the $18.5 million construction of the Dubbo Regional Theatre and Convention Centre.

The upgrade of the Dubbo Sewage Treatment Plant was the successful recipient of a 3 per cent interest subsidy under the NSW government's Local Infrastructure Renewal Scheme.

Council applied for a 3 per cent interest subsidy on $12 million of the original borrowings required for the project and was successful, which will see $1.9 million of savings across the 10-year loan.

An average of 35 staff are on site at the plant on a daily basis, with a maximum of about 40.

Annual charges for sewerage services were increased by 6 per cent at the start of the financial year from $651.87 to $690.16, while the drainage charge was increased by $1.94 to $86.60.

The increases were to ensure the operating expenses, infrastructure maintenance and construction needs could be met for the growing population.