Environmentalists say plans to explore for oil just five kilometres from Western Australia's Ningaloo Reef will affect the health of marine life in the region.

It is part of BHP Billiton's efforts to expand its $1.7 billion Pyrnenees project, which began producing oil in 2010.

The Department of Environment says it is assessing the proposal to determine whether it will have a significant impact on the World Heritage-listed reef.

But World Wildlife Fund WA director Paul Gamblin says it is highly likely the exploration will affect marine life on the reef.

"It's concerning that there's a lack of an overarching plan or vision from government as to what will happen for places like Ningaloo and the places like Ningaloo," he said.

"What is the future to look like? Are we going to see oil and gas encircle the reef?

"It's very concerning, this latest development is a further indication that Ningaloo is becoming increasingly an oil and gas destination, a Mecca for oil and gas, whereas it's one of the world's last healthy coral reefs and it needs to be fully protected."

Mr Gamblin seismic surveys, which are used to locate oil and gas reserves, emit high-decibel impulses which can affect the health of marine life.

"BHP is proposing to create a whole lot of very intense underwater noise, seismic activity, which can disturb blue whales, humpback whales, turtles, and other animals that use this area," he said.

"It's a marine superhighway."