An APN investigation has also confirmed details of 35 oil spills and the "unplanned dumping" of more than 4200 cubic metres of dredge spoil in the harbour during the Australia's biggest-ever dredging project.
The Observer can reveal a total of 48 environmental incidents, between July 2011 and October last year.
Most of the incidents have remained hidden from public scrutiny - some for more than two years - as neither the Gladstone Ports Corporation nor the Federal Environment Department is required to report details publicly.
And another nine incidents have occurred on the project since then, but the department refused to release the details.
The sewage release occurred from the cutter suction dredge Al Mahaar in October last year, after a blockage in the sewage treatment system.
Documents obtained under Freedom of Information laws revealed "untreated sewage mixed with grey water" was released to prevent "a likely emergency situation" on board.
Subsequent investigations confirmed the Al Mahaar did not have a back-up system, despite being certified as having one under international standards.
"The availability of such a facility would have allowed the transfer of waste water into alternative storage facility," the incident report reads.
Documents seen by The Observer confirmed the lack of "an optional discharge line and connectors", showing the Al Mahaar did not meet the certification standards at the time of the incident.
A Gladstone Ports Corporation spokesman said the GPC had ordered an independent inspection by a "third party independent surveyor" and port staff had also "verified all paperwork" for the vessel prior to deployment.
But while the spokesman said the vessel was "found to be fully compliant" with the International Sewage Pollution Prevention Certificate, none of these checks identified the missing discharge line before the Al Mahaar began work.
Contractors have since installed a holding tank on the Al Mahaar, with all sewage to be treated on land.
The new storage tanks on board mean the Al Mahaar does now meet the standards, with the dredger still operating on the project 10 months after the incident.
Documents show the incident was rated as a rare temporary nuisance with "no permanent damage" to the environment.
A spokeswoman for the Commonwealth Environment department said the port corporation had "adequately reported on the actions it has taken to remediate incidents".
Western Basin facts
- Contractor: Van Oord - Dredging International Joint Venture
- Proponent: Gladstone Ports Corporation (responsible for environmental management)
- Began: May 20, 2011
- Dredges: Nine dredges have worked on the project, with two currently operating - Al Mahaar and Athena
- So far: 18.9 million cubic metres of sediment has been dredged
- Total: Up to 46 million cubic metres of sediment approved to be dredged
- Project cost: $1.3 billion
Total 35 oil spills in 14 months hit harbour in dredging operation
DETAILS of more than 30 oil spills in water and on land during the huge dredging operation underway in Gladstone harbour have come to light for the first time.
While confidential incident reports on the 35 spills have highlighted the risks of the project, the documents have also shown a speedy response to help prevent any lasting damage.
The oil spills largely involved failures of hydraulic equipment on dredgers operating on the Western Basin Dredging and Disposal Project.
Dating from July 2011 to September 2012, the oil spills totalled more than 900 litres of hydraulic oil, diesel and other fuels spilt into Gladstone Harbour, the Western Basin Reclamation Area and surrounding land.
Most spills involved a small amount of hydraulic oil, ranging from less than a litre of oil spilt to up to 20 litre of oil spilt into the harbour waters.
But one incident on the vessel BHD Big Boss in November 2011 showed 575 litres of hydraulic oil was spilt on the deck with about 275 litres spilling into harbour waters before being cleaned up.
Despite the high number of minor spills on the project, the reports claim every incident was met with swift response, including using absorbent pads and booms during clean-up operations.
A Gladstone Ports Corporation spokesman confirmed all equipment was checked internally by a third party, and where the port had any concerns, instructions were given to replace all hoses on the equipment.
The spokesman said while any spill was highly regrettable, the port and contractor had "worked to ensure all spills are reported and improvement plans put in place to limit future incidents".
A spokeswoman for the Federal Environment Department said the department did not identify any breaches of approval conditions during investigations earlier this year.
Nine dredging incidents
A SERIES of machinery malfunctions and human errors caused more than 4200 cubic metres of sediment to be discarded in "unplanned dumping" incidents in Gladstone Harbour during the dredging project since November 2011.
But only one of the dumping incidents resulted in a fine from Commonwealth regulators, as the other incidents were not a breach of approval conditions.
The only "unplanned dumping" incident found to breach conditions was self-reported by Gladstone Ports Corporation
Federal Environment Department documents obtained under Freedom of Information laws reveal the nine incidents between October 2011 and June last year resulted in 4232 cubic metres of sediment being dumped outside the approved East Banks Disposal Site.
The incidents were primarily caused by machinery malfunctions, where dredge hopper doors were not completely closed, or by human errors at the control panel.
Eight of the incidents involved sediment being dredged, dumped back into the same area and dredged again. These were allowed under approval conditions.
A department spokeswoman confirmed the incidents were not a breach of the approval conditions.
The only "unplanned dumping" incident found to breach conditions was self-reported by Gladstone Ports Corporation, resulting in a $6600 fine for an emergency dump in January last year.
A port spokesman said the incidents did not impact areas outside of the dredging footprint.
In June this year, GPC carried out its own investigation into a dirty water leak from a Western Basin dredge bund and into the harbour.