He said suggestions to pipe stormwater away from surging surf and scouring gravel would require extra cost.
"Pumping solutions were not considered to be financially viable due to the significant immediate capital cost and ongoing annual operational costs."
When constructed at the end of last year the platform disappointed some people who had made the false assumption it would be a pier extending into the sea.
Mr Jack said that would have been a waste of money.
"To build a longer outfall structure at this time would have been prohibitively expensive due to the challenging nature of the site and significant associated construction costs and would have provided no additional operational benefit."
The council was "very familiar" with the challenges faced "from a long history of operating and maintaining stormwater outfalls along this section of Hawke Bay", he said.
"This is a difficult environment to design, construct and operate in and any design would face similar challenges and are complicated further by the low ground levels inland of the beach foreshore."
Napier mayor Bill Dalton waded into recent social media criticisms, defending the stormwater upgrade.
"The pipe under the viewing platform is not yet connected to the Napier stormwater system," he said.
"Therefore it has no outflow to assist in clearing the pipe.
"With any such outfall into the sea, there is a degree of experimentation to achieve the final and long-term solution.
"One possible solution is a right-angled exit pipe heading north. We will fine tune the final exit point and end up with one that works."
The viewing platform cost $716,000 and the outflow pipe $684,000, from a $2.4 million budget for a CBD stormwater upgrade.
