SYDNEY (AFP) - Australian police apologised on Thursday after a suitcase with plastic explosives inside used for sniffer dog training was left at Sydney airport and only found four weeks later by a passenger.

The woman, travelling through the country's busiest airport on Tuesday, was given the unclaimed bag as a replacement after her own bag was damaged during a flight. The airline that gave the woman the bag was unaware that it had been lined with explosives

When she reached home and opened the suitcase, she found 230 grams of plastic explosive in a training device and took it to a local police station in Cessnock, about 150 kilometres from Sydney.

Officers evacuated the building as a precaution and traced the explosives back to the Australian Federal Police.

Police said the device was not live and was inadvertently left at the domestic terminal after a detector dog training exercise on Aug 14.

"Police takes this error seriously and the canine instructor who inadvertently left this device behind has been identified and will be the subject of a formal professional standards investigation," federal police Sydney airport Commander Wayne Buchhorn said.

"Although the travelling public was not in danger at any time, we regularly review our processes in this area, and we will do again in the light of this incident."

Police dogs are a routine sight at Sydney airport, sniffing passengers, bags, vehicles and the terminal buildings for illegal drugs, explosives and currency.

"This type of training is essential for our canine teams, allowing the dogs to experience potential threats in a real environment," police added.