Mr Johnson explained they sought a WCO over a regional plan as it gave primacy to primacy to in-stream ecological, amenity and cultural values.
It was also heard by a tribunal with higher standing than "regional politically driven" councils, and unlike a regional plan which had a limited term, a WCO would stand until repealed, meaning it offered an "enduring life of protection".
He warned waterways were a finite resource which needed to be protected as the country's population, and industry grew.
"There are only so many of them, and each time some "use" is made of one, or part of one, the overall stock of what was naturally there before is diminished."
"With the passage of time, it is easy for the casual observer to forget, or not realise, that the flow and, or instream quality of a water body before them is significantly less than it was originally."
Commercial water users were "involved in a progression of increasing use for which they have to date never been prepared to say 'enough is enough'," he said.
However, tribunal member Alec Neill asked Mr Johnson whether the issue instead required different parties working co-operatively together.
Mr Johnson repeated earlier statements that communities could not expect to keep taking water, and "we have to face up to the hard reality of that".
"There are limits and community needs to step up to that."
Fish and Game's role in the local-TANK planning process was also discussed - although Hawke's Bay Fish and Game Council had been involved since its inception, Mr Johnson said they felt the WCO was a more appropriate tool to provide protection.
In his evidence Mr Venman noted TANK's process had been slow, picking up only after a re-launch. Since then, there had been "substantial" progress, with its findings confirming the important values of the Ngaruroro.
He disputed criticism the WCO was an attempt to "derail" TANK, as it was initiated prior to TANK's establishment.
He noted their agency still supported TANK, and "I believe there is an opportunity for the two processes to benefit each other".
As part of their evidence the pair also discussed the political history of WCO law, the Upper Ngaruroro River rainbow trout fishery, and fishery values.
Before the hearing ends in early December, presentations will also be heard from a range of parties including several councils, the Department of Conservation, Hawke's Bay primary sector representatives, local business owners and recreational groups.