That included waste produced by a large number of fish in a confined area, and uneaten food.
The fish would be fed modified vegetable and animal material, most of which would be imported.
Mr Keys said the company's response when asked how it would manage waste was that once the immediate area was polluted the cage would be moved to allow tides to wash the buildup into the harbour.
He was also sceptical about claimed employment potential.
Labour MP Shane Jones has backed the runanga's plans, saying something had to be done to replace the 66 jobs lost when Sanford closed its Kaeo oyster plant.
However, Mr Keys believed the fish farm would lead to only five permanent jobs, four of which would go to outside experts.
Given the virus affecting the oyster industry in Northland, he doubted the oyster farm would create new jobs.
It made no sense to risk spoiling one of Northland's most spectacular harbours when tourism was the region's biggest earner after pastoral farming, well ahead of aquaculture in both jobs and income.
Tourism employed roughly 7000 Northlanders and made $590 million a year, compared with aquaculture's 330 jobs and $19m.
The steering group, which aims to protect the harbour from commercial fishing, reclamation and other commercial exploitation, claims to have the support of 95 per cent of local residents.
Northland Regional Council aquaculture rules (Plan Change 4, which is still wending its way through the Environment Court) declare Whangaroa Harbour a no-go area for new aquaculture developments, but the Whaingaroa Fisheries Company argues that its original application was made in 1996, before Plan Change 4 was introduced.
Yachting New Zealand has also submitted against the proposal, saying it would take up close to half of Owanga Bay and block safe access to some of the harbour's best anchorages.
It would be hard to get past the farm in high winds as boats would have to be virtually on the rocks to get through the narrow passage, and if the wind shifted at night, changing anchorage would be "a nightmare".
The fish farm would also cover a recreational fishing ground and a scallop bed, albeit one depleted by dredging.