The $23.742 million upgrade is the largest single capital project Dubbo City Council had undertaken.

As part of the work, two bioreactors have been constructed, 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 the plant would be tested with water to make sure the whole system worked.

Mr Watson said the project was two or three weeks ahead of schedule and further tests would be done throughout the month before sewage was introduced in September.

Technical experts from Sydney would be visiting the facility on August 24, he said.

Mr Watson said the old sewage basins would not be destroyed, but could be used for overflow or sludge storage.

Dubbo mayor Mathew Dickerson, Dubbo City Council general manager Mark Riley and council's manager works and services Ian Bailey were given a run down of plant's progress during a tour on Tuesday.

The Dubbo Sewage Treatment Plant receives and treats all domestic and trade waste from the Dubbo urban area. Once completed it will have sufficient capacity to cater for a population of 55,000 people, 15,000 more than the current plant, to cater for the city's growing population.

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 construction is being completed by Cockram and supervised by NSW Public Works.