Thirty Vietnamese found illegally fishing in Australian waters were on Tuesday handed suspended jail sentences and had their boats destroyed in what authorities said was a strong deterrent message.

The crew were from two boats caught illegally fishing in a Coral Sea marine reserve off Australia's north coast on June 2, with diving gear and six tonnes of sea cucumber -- a delicacy in countries such as China -- found on board.

The fishermen all pleaded guilty in a Darwin court to breaking Australian fisheries and environmental laws.

Their penalties included suspended jail sentences ranging from two months for the crew to five and seven months for the masters of the vessels.

They were also issued good behaviour bonds ranging from two to three years, with up to Aus$2,000 (US$1,477) to pay if they are breached.

"Illegal fishing threatens the economic viability and sustainability of Australia's well managed marine resources," said Australian Fisheries Management Authority general manager Peter Venslovas.

"The convictions and destruction of the vessels are a good result and will send a very strong message to all those considering illegally fishing in Australian waters."

The case came on the same day a Papua New Guinean boat was apprehended for suspected illegal fishing in Australian waters, allegedly carrying sea cucumber and two shark fins.

According to a United Nations Development Program report this month, up to 26 million tonnes of fish is caught illegally each year.

Stepped up surveillance by Australia has seen the amount of illegal fishing fall from highs of 367 boats caught a decade ago to just 17 so far in the 2015-2016 financial year, government data shows.