Qantas emits more carbon dioxide per passenger-kilometre than any other airline operating across the Pacific, according to an analysis by the International Council on Clean Transportation, the group that exposed the Volkswagen emissions scandal in 2014.

For each kilometre Qantas transports a passenger across the Pacific, it uses 64% more fuel than the two most fuel-efficient airlines operating across the Pacific: Hainan Airlines and All Nippon Airways (ANA).

One litre of aviation fuel was able to transport one passenger just 22km, on average, on Qantas flights, while Hainan and ANA were able to take a passenger 36km on 1L of fuel.

The wide variation between airlines provides a stark contrast to the industry’s aspirational goal of improving fuel efficiency of international flights by just 2% annually.

Aviation currently accounts for about 2.5% of global carbon dioxide emissions. But pollution from the industry is expected to increase and, by 2050, use up a quarter of the allowable greenhouse gas emissions if the world is to keep global warming at less than 1.5C – the world’s “carbon budget”.

While Hainan and ANA tied as the most fuel-efficient airlines, they used different strategies to achieve the relatively low fuel burn.

Hainan Airways, a Chinese carrier, employed more fuel-efficient aircraft, using the Boeing 787 Dreamliner for more than 80% of its flights, and the Airbus A330 for the remainder. It also avoided having excessive empty seats, further increasing its efficiency.

Meanwhile, the Japanese airline ANA achieved the low fuel-burn by carrying a lot of cargo, which the analysis treated as equivalent to carrying people.

In contrast, Qantas used the two most fuel-intensive aircraft – Airbus A380 and Boeing 747-400ER – and carried the most empty seats of any transpacific airline, filling just 74% of its seats.

Comparing all the airlines, ICCT found that how much freight an airline carried was the strongest determinant of its fuel efficiency, explaining 48% of the variation between airlines. Seating density, aircraft efficiency and passenger load were the other major factors.

Looking at specific routes, the analysis found Delta was the most fuel-efficient airline on the Sydney-Los Angeles route, flying a passenger 33km on a litre of fuel. Virgin Australia and United were not far behind on 32km, while Qantas lagged on just 24km.

The most competitive transpacific route was Los Angeles-Tokyo, with six airlines flying between the two hubs. On that route United was the most fuel-efficient, taking a passenger 42km on a litre of fuel. Singapore Airlines was the least fuel-efficient, taking a passenger just 26km on a litre of fuel.

The report notes that Qantas is planning on using eight new Dreamliners for flights to the US, which, if operating now, would lift the airline one place from the bottom, above Korean Air and tying with Asiana in the second-bottom place.

The results follow an earlier analysis ICCT conducted on the efficiency of airlines on transatlantic routes in 2015, which found a significant but smaller variation between airlines than was found on Pacific routes.

In that report, British Airways and Lufthansa were found to have emitted 51% more carbon dioxide than the cleanest flyers, Norwegian Air.

“This research shows that there are a variety of ways that international airlines can reduce fuel use and carbon emissions,” said Brandon Graver from ICCT, the lead author of the study. “Buying new aircraft, carrying large numbers of passengers and optimising freight strategies all make a difference.”

Dan Rutherford, ICCT’s aviation program director and co-author of the paper, said: “There’s a reason airlines around the world are starting to avoid very large aircraft like the 747 and A380.

“Newer twin-engine widebodies provide the payload and range capabilities needed for transpacific flights with much lower fuel burn.”

Alan Milne, head of fuel and environment at Qantas told the Guardian that the airline ranks low in the analysis because it uses large aircraft, flies long distances and has premium cabins that leave more space between passengers.

“We’re committed to reducing carbon emissions and continually look at ways to lower them across our operations,” he said. “We are switching our 747s for more fuel-efficient Dreamliners and we have several data-driven programs in place to reduce fuel burn.”