The fleet of brightly coloured wooden Vietnamese fishing boats that the Australian government will use to turn back asylum seekers are unseaworthy and ethically wrong to use, marine sources in Darwin say.

A government official and a local fisher say they were shocked at the building standards of the 20-metre green, blue and red fishing boats that will replace the garish orange lifeboats that were deemed "unsinkable".

Two of the Vietnamese fishing boats in a Darwin boat yard. Credit:Glenn Campbell

"The first boat the shipyard launched sunk, and it had to be retrieved with a crane," the government worker said.

"The materials are a really poor quality as well as the craftsmanship. They are completely unsafe and not a passenger boat.