It remains a "mystery" how a 1.5 metre bronze whaler entered a $200,000 enclosure designed to keep swimmers safe from sharks at a WA beach, according to authorities.

It took eight hours to remove the intruding shark after a swimmer about to enter the beach enclosure at Albany on November 24 spotted a dark shadow lurking in the water.

A week-long investigation into the shark incident has left Albany council and the builder of the experimental safe swimming barrier both baffled.

Divers have inspected the 320 metre length of barrier, which drops in places to a depth of six metres, and fences off a large swimming area the size of two football fields.

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Divers have inspected the 320 metre length of barrier, which drops in places to a depth of six metres, and fences off a large swimming area the size of two football fields. (Global Marine Enclosures)

"There was no obvious way the shark could have swam into the enclosure. How the shark came to be in the enclosure remains a mystery," City of Albany's Executive Director Infrastructure and Environment Matthew Thomson told nine.com.au .

"There is no evidence the shark breached the enclosure so we are still looking at possible causes."

Mr Thomson said a dead 1.5 metre mako had also been found inside the Albany enclosure last month, an unusual incident he described as "odd".

"We suspect [the mako] was dumped in there as a closer inspection found the shark had sustained injuries consistent with fishing.

"The enclosure has been in place for almost three years and performing as designed so it is odd that we have found these two small sharks within the enclosure in the past two months."

The performance of the anti-shark enclosure will be discussed by the council when its three-year trial ends in 2019.

The performance of the anti-shark enclosure will be discussed by the council when its three-year trial ends in 2019. (Global Marine Enclosures)

"We will be working with the manufacturer to review its performance before the council considers future options," Mr Thomson said.

The Perth-based company which built the barrier, Global Marine Enclosures, has a second enclosure trial in operation at Quinns Beach, Perth.

The Quinns Beach barrier extends 85m offshore and runs 300m along the beach. The WA State Government funded barrier, launched in January 2017, will be tested over a five-year period.

Edward Khoury, owner of Global Marine Enclosures, told nine.com.au he was shocked when told the whaler had entered the Albany barrier.

"We have no idea how the shark came to be inside the enclosure," he said.

Mr Khoury said he was surprised the whaler had somehow made its way into the enclosure.

Last month a 20-year-old young bodyboarder was bitten on the leg by a shark at Pyramids Beach, near Mandurah, south of Perth.