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An energy giant has been accused of covering up the severity of an oil spill which allegedly killed dolphins off Western Australia (WA) last year.

A whistleblower's statement read out in parliament this week alleges dead dolphins were found floating near the oil slick caused by Santos last March.

Santos has previously denied the deaths were connected to the spill from its Varanus Island facility.

The Australian firm is yet to respond to request for comment.

The company last April told the WA Today newspaper the incident was a "minor spill" with "negligible" environmental impact.

In November it added that the dead dolphins had been sighted "a couple of hours after" the spill, arguing it would have been too early for their deaths to result from it.

But the whistleblower claimed the company couldn't know for sure as it did not send environmental experts to the island until more than a week after the incident.

The statement from a former Santos employee was read out by independent Senator David Pocock on Thursday.

The employee said the Lowendal Islands, some 75km (46 miles) off the WA coast, is known for "pristine white sand beaches", "gorgeous blue turquoise water" and "abundant marine and bird life".

But in the statement he said sea snakes had "writhed in agony" and marine life had suffered after some 25,000 litres of condensate leaked from an underwater hose.

"I was then shocked at the public comment from Santos," the whistleblower said.

"Tens of thousands of litres of oil in the ocean, dead dolphins and sea snakes. How was this negligible?"

They added that Santos' alleged delay in sending environmental assessors was a breach of the company's obligations.

"They could not have known the real scale of impact, it was never checked," their statement read.

Santos' conduct was "deceptive" and "contrary to its internal code of conduct… and, possibly, the law", they said, adding that other employees had also spoken up internally.

Mr Pocock, who tabled the statement, said he found the testimony and footage "very distressing".

The incident raised major questions about marine environment protections and Santos' conduct, he said.

The spill is the latest mark in oil company's "very poor environmental and safety record", WA's peak conservation body told the ³ÉÈË¿ìÊÖ.

"It is vital that regulators make an example of Santos. Petty fines are simply not enough to prevent this sort of thing from happening again," Maggie Wood said.

She also expressed concern that the allegations had only come to light a year after the incident, and only because of a whistleblower.

"How many incidents like this might we not know about, simply because the likes of Santos neglected to tell us?"

Santos has not yet issued a public response and has not replied to the ³ÉÈË¿ìÊÖ's queries.