Frankston Magistrates Court was last week told the man, 30, from Noble Park, and several other people were watched as they unloaded the net at Patterson River in the early hours of Christmas morning.

The court heard the defendant was planning to set the net in the waters of Port Phillip Bay. He was arrested by police officers and later admitted the net was his, that he had owned it for six months and that this was the fourth time he had used it.

The net was later measured and found to be about 65 metres long with a drop of three metres and an eight-centimetre mesh.

Magistrate Rodney Crisp said a significant penalty was needed to discourage such activity and convicted and fined the man $4000 on charges of using and possessing commercial fishing equipment.

Fisheries officer Sean Brodie said fisheries managers are concerned at the increased frequency of illegal netting detected in the past six months along the eastern shores of Port Phillip.

"These types of illegal nets catch indiscriminately and have no regard for fish size and bag limits," Mr Brodie said.

"They can catch a large amount of fish in a short period and, when left unattended, can kill marine birds and mammals. They pose a serious threat to the life in our bay."

Anyone who sees or suspects illegal fishing activity is urged to call the 24-hour reporting line 13-FISH (133 474).