Gladstone Port Corporation has revealed a black substance found in the harbour to be coal fines.
The corporation investigated the spill after a concerned local photographed the dark substance.
Gladstone Ports Corporation (GPC) said it could not indentify what caused the coal fines in the water.
Gladstone Observer reports a GPC spokeswoman said the material was suspected to have come from one of the wharf’s sample plants.
"Whilst situated on the wharf, (they) are not operated by GPC," she said.
GPC added that it had invested $5.4 million to commission in a phased approach to seal the sample plants along the wharf and that every report of contamination was investigated and recorded on a management system.
Maritime Safety Queensland said the spill was not officially reported.
A spokeswoman for MSQ said public reporting of incidents was an important in indentifying pollution in the harbour.
"Once an incident is reported, GPC will act as their first strike response and report back to MSQ in order for them to react appropriately," a GPC spokesperson said.