The department said the leakage, which occurred about 10:45pm on Sunday, had the potential to impact local marine ecology and damage vessels that come into contact with the chemical.
"Officers are continuing to investigate the exact amount of sodium hydroxide [caustic soda] spilled into the harbour, but initial reports suggests a loss of several tonnes," it said in a statement.
"At this stage, it is believed that the most likely area for any impacts to occur is to the south of the QAL wharf, because the release occurred at high tide."
No signs of environmental damage, QAL says
A QAL spokesperson said they discovered the discharge had occurred while unloading caustic material, with authorities notified immediately.
"Caustic is pumped from vessels into storage tanks through large pipes," the spokesperson said.
"It is believed a portion of this flow has bypassed into the seawater flushing lines and into the ocean.
"The discharge was identified as part of unloading monitoring processes.
"The system was immediately shut off.
"Extensive monitoring of the harbour and downstream areas has taken place overnight and today and has not shown any signs of environment damage including changes to pH levels and impact to fish and ocean species.
"Caustic is used at QAL to assist in the extraction of alumina from bauxite.
"Caustic neutralises when mixed with seawater."