The Nature and Biodiversity Union (NABU) is lambasting the cruise industry for running ships on heavy fuel oil, without recognising the sector's long-term plans to lower exhaust emissions.

"It is scandalous that in 2018 there are still ships coming into the market which are built to use heavy fuel oil as fuel," NABU chief executive Leif Miller said.

NABU put LNG-powered AIDAnova atop its 2018 ranking of vessels, while saying LNG emits the same level of greenhouse gas as diesel and calling out other operators as "dirty".