Although he touched on other values, Mr Kay stressed the river's high water quality and habitat meant the Ngaruroro catchment was home to several native fish and birds, many of which were at risk of extinction.
"Forest and Bird believe it is these characteristics, and those that will be covered in the evidence of our co-applicants, that make the Ngaruroro River nationally outstanding, and therefore worthy of the recognition and protection provided by a WCO," he said.
Forest and Bird supported protecting the upper catchment, as flows would be maintained at a suitable level to ensure the protection of indigenous species and their habitats.
Protection would also ensure the river's water quality was maintained "from source to sea", and mean the natural character of the area was maintained.
The organisation had considered various threats which could pose a danger to these values - from intensification of land use, to fish barriers, or dams.
When asked by the tribunal, Mr Kay said he was not aware of current proposals which would threaten values in the upper river.
"But in saying that I'm not naive in thinking that any of those could become a threat to the values of the upper river in the future.
"You only have to look around New Zealand at what we've done to some of our other amazing rivers to know that those kinds of threats are not unimaginable."
There has been criticism the WCO seeks to usurp the local Tank-planning process, with some of the applicant's local representatives - including Forest and Bird - having been stakeholders in the project.
"Our decision to join the co-applicants for a WCO does not in any way reflect on a
lack of commitment to, or a criticism of, the Tank process," Mr Kay said.
"We see the recognition of the nationally outstanding values of the Ngaruroro River as complementary to, rather than in conflict with, the regional planning process that Tank is undertaking."
He told the tribunal Forest and Bird was a long-standing advocate for the maintenance and protection of freshwater quality in Hawke's Bay, such as their involvement against the Ruataniwha Water Storage Scheme.