He said pressure on the Hill St intersection would be increased by extra traffic from Warkworth and the eastern beaches having to head north through it, to reach the new motorway, which the agency hopes to complete by about 2021.
Difficult right-hand turns from the east, into the existing main road north, would become even harder and "the intersection will not cope," he told the five-member board chaired by retired High Court judge John Priestley, QC.
Only a major upgrade - either entailing bringing Warkworth's main Elizabeth St directly into the intersection as proposed in 2009 by the agency or a radical new design he has prepared - would allow it to keep functioning in the meantime.
"I believe there are no minor tweaks - forget it."
Mr Parker said earlier that a major intersection upgrade now would cause "chaos" to traffic passing through Warkworth over two summers, but has proposed widening three approaches to it in the meantime.
He said Mr Williams' design would mean an incursion into Kowhai Park, which he believed would be opposed by many local residents.
But Mr Williams said the Rodney Local Board backed his proposal, which could be built largely off the existing SH1, in less than a year.
He also wants the Transport Agency to build a motorway interchange south of Warkworth, so drivers heading to or from the town will not have to loop around the north.
The only interchange the agency proposes between Orewa and Warkworth is at Puhoi, where it plans south-facing ramps only.
Heavy Haulage Association chief Jonathan Thomson asked the board to require the agency to add north-facing ramps to allow trucks carrying over-size loads to join the motorway from the Hibiscus Coast Highway, to which they are now restricted by being barred from the Johnstones Hill tunnels.
The association also wants the motorway built wider than proposed, to cope with over-size loads he said were become more common because of a trend toward pre-fabricated construction.