Dangerous chemicals destroyed by police this morning are believed to be highly-volatile explosives.
It is understood a passerby found what is believed to be several kilos of the explosives in the Leschenault Estuary in Australind yesterday afternoon.
Police divers are searching the area to make sure there are no more packages.
Organised crime squad detectives were called in and initially believed the substance in the packages were drug-related.
But when officers returned to their city office and began examining the items for testing they realised it was something more volatile and were forced to destroy it during a major operation this morning.
Police are hunting the people who hid the explosive material, which is believed to be a powder, and will not speculate on the motives behind what they were planning to do with it.
Counter-terrorism, State Security and major crime squad officers are involved in the investigation and it is understood they have not ruled out links to bikies or terrorists.
Acting Cdr State Crime Scott Higgins confirmed the material found could be explosive and was not known to be used as a drug, but he would not confirm what the chemical material was and how volatile it was.
Police closed roads and evacuated buildings this morning as they moved the explosives to Gloucester Park, where they were blown up this morning.
The delicate operation began at 4.30am.
The substance was being held as evidence but when senior police learned of their location they ordered their immediate removal and destruction.
All track work at Gloucester Park was suspended.
Drivers were told to avoid the areas between Curtin House in Beaufort Street, Northbridge, and Gloucester Park because road closures will hamper morning traffic.
Beaufort, Newcastle, Lord and Royal streets were affected.
Buses that travel past Curtin House were diverted.
Officers from the bomb squad and the fire brigade were at the ground.
St John Ambulance officers were also involved in the operation.
It is understood Gloucester Park was chosen because it was the nearest open area.Police did not want to transport the chemicals too far because of their volatility.