A deceased blue shark has washed up in the Ahuriri Estuary, just over a week after wastewater was released into the Purimu stormwater stream, which flows out to sea through the estuary.
The 1.5m (5ft) shark was spotted by Pete Malesis while he was on a walk on Thursday.
Shark expert Clinton Duffy said while not uncommon in Hawke's Bay, the blue shark (Prionace glauca) is rarely seen in enclosed waters.
"They're an oceanic species, so it is unusual to find them in enclosed waters like Ahuriri Estuary," he said.
"It may have been injured and straggled into the estuary, or it may have been discarded from a fishing vessel."
Following the recent deluge, Napier City Council released pressure from the wastewater network and discharged wastewater into the Purimu stormwater stream, which flows through the Ahuriri Estuary.
A National Aquarium of New Zealand spokeswoman said pollution is an unlikely factor in how the shark ended up in the estuary.
"It may have come in on full tide and become stranded as the water receded. Another possibility is this is a discarded fishing catch, perhaps even washed in from outside of the estuary," she said.
"It's less likely that this finding is related to the recent flooding or a pollutant in the water."
The spokeswoman also said a shark like this would not typically be found in the Ahuriri Estuary, which largely drains when the tide is out.
About 66 types of sharks are found in New Zealand waters, ranging in size from the tiny pygmy shark which grows to 27cm long to the 12m whale shark.
Duffy said several species are protected in New Zealand waters, so it is always best to report them if found stranded.
Anyone who finds an injured, entangled or dead protected shark is encouraged to report it to the Department of Conservation.