A giant yellow inflatable duck believed to have been missing at sea off Western Australia for almost a week was actually found making a break for Rottnest Island by a local fisherman just hours after being swept away.

"Daphne" the duck was spotted by Toby Gibb near Rottnest after he went fishing with his mate early on the morning of March 11.

"I found it an hour after it went missing," he said.

"No-one knew about a missing duck at that point, we didn't know where did it come from.

"It was a little over 30 kilometres offshore … probably from about Fremantle harbour point. It was heading west, going along the southern side of Rottnest Island."

Daphne was meant to be this year's mascot of the annual Coogee Jetty to Jetty Swim, organised by the Cockburn Masters Swimming Club.

The duck was blown into the Indian Ocean off Coogee by strong winds around 5:30am on March 11, before the swim began.

The alarm was raised by the club after Daphne's prolonged disappearance, leading some to speculate whether the duck had set sail for the Indian Ocean.

But Mr Gibb said Daphne had spent the past week recovering at his house in the southern Perth suburb of Gosnells and would soon be handed back to organisers.

"Our intention was trying to take it back to the shore," he said.

"It was going to take a long time to get all the air out of it because it's so big, so we tied it to the back of the boat."

'Like winning lotto', duck's owners say

Mr Gibb said Daphne was in good condition when he found it.

"It looked perfect. It was fully inflated, there was no damage to [it] whatsoever," he said

Daphne the duck was meant to be the mascot for this year's Cockburn Jetty Swim. ( Supplied: Cockburn Masters Swimming Club )

But Mr Gibb said he did not know who the duck belonged to or who to contact at first, and he travelled to Melbourne after the fishing trip before returning to Perth almost a week later on Saturday, when the club contacted him.

Cockburn Masters Swimming Club president Peter Marr told ABC Radio Perth the club was aiming to pick up Daphne on Wednesday.

"It must be like winning lotto and you've checked the numbers on your ticket but you haven't been to the newsagents to cash it in," he said.

"We are now in the delicate negotiations and we want Daphne back."

Daphne cost the club $900 and is the size of a small caravan.

It is not the first time a giant rubber duck has drawn attention from the media.

Oversized inflatable replicas of the classic bath toy designed by Dutch artist Florentijn Hofman have been floating around the world's most famous landmarks since 2007 — attracting big crowds and spawning many replicas.

The five-storey duck made its grand entrance in Sydney's Circular Quay and Darling Harbour in 2013.