It seems the veterinary experts were on the money when they diagnosed why a sea lion at Adelaide Zoo was not eating and was losing weight.

Tasko is now $1.10 lighter, after surgery to remove seven coins from his stomach.

The zoo says the sea lion may have mistaken the coins for food when they were thrown into the pool of his exhibit.

Senior veterinarian Ian Smith says Tasko went for days without eating, slept more and lost 30 kilograms before the coins were found and removed.

The diagnosis required use of specialised medical equipment because the coins had not shown up in initial X-rays.

They were hidden behind the other items in Tasko's stomach.

Dr Smith says several stones also were found.

He says usually they are not a problem for seals, but they were in this case.

"The coins are quite well worn, they've been ground down by the stones and potentially releasing metals into his system," he said.

Dr Smith says it was fortunate all the coins were Australian because foreign coins can contain toxic metals, which would boost the risk of ingestion proving fatal.

"We couldn't actually sort of read what was on them so they'd certainly been well worn and well ground down," he said.

Some of the worn coins after they were removed from the sea lion's stomach

"On the whole, Australian coins are probably one of the safer coins to swallow because they don't have any zinc in them compared to some other coins from overseas and zinc was one of the main metals we were sort of concerned about."

Tasko is now getting back into training and eating well.

Signs have been put near the sea lion enclosure asking visitors not to throw anything into the area.