"This water is sourced from a number of industries including tanneries, meat processing and rendering," she said.
Hawke's Bay District Health Board medical officer of health Nick Jones said although the degree of contamination is not yet confirmed, people should not to swim in Pandora Pond for at least three days.
Council staff took samples of the discharge upstream and downstream, as it was unclear if it had made it into the stormwater system and travelled as far as Te Whanganui-a-Orotū (Ahuriri Estuary).
"It is unclear at this stage what the blockage was," the spokeswoman said.
Tangata whenua were informed of the incident, along with the Hawke's Bay Regional Council, HBDHB and the Ahuriri Estuary Protection Society.
Investigations into the cause and significance of this incident are ongoing and temporary signs have been erected in the area to notify the public.