Vitali collected around 10 kina while enjoying his sunset dive but threw back the three smallest kina and swam back to the shore.
When he looked in the catch bag the key was gone, then he "clicked".
"One of the little buggers had sucked onto my car key and took it with him."
The geotechnical engineer swam back out to see if he could see his stolen car key but had no luck.
He left the beach with no car key but was met by his partner Erica Colley who brought a spare key down to him.
Vitali posted the tale of "the kina who stole my car key" on a local community Facebook page which delighted many people.
The post gained more than 240 likes and many comments from people saying they would keep an eye out for the key.
"Someone even said they were interested in writing a children's storybook," Vitali said.
But he was interested in possibly creating a book himself as he was keen on drawing.
Vitali had since gone back to Leisure Island to search for the key.
"I think it might wash up eventually, it could just be buried under some sand."
And to the kina who stole the car key, Vitali said: "his friends were really tasty".
The kina and the car key
Who- Australian man Kim Vitali
What- Went diving and a kina latched onto his car key before it was thrown back
Where- At Shark Alley, to the right-hand-side of Leisure Island
How can you help- If you spot the key, make contact via the Mount Maunganui Noticeboard Facebook page.
