Construction is already well-advanced, with the former wooden boathouse now having large steel supporting structures and two levels with concrete floors.
However, since the decision, Duke and his wife Patricia have appointed a new lawyer, Richard Brabant, who asked Auckland Council for permission to carry out "essential work required to make the structure secure and watertight".
This includes framing up the walls of the front deck, putting in a temporary roof pitch over the front deck, and cladding the roof and window openings.
Last week, the council's compliance monitoring team made an "in-house" decision saying the work was "reasonable" and allowed it to go ahead.
Marine environment campaigner Andy Coleman, who is spearheading the court action against the helipad project, is angry. He says the council should have declined the Dukes' request.
Coleman, who heads the Kawau Island Action coastal protection group, argues the council has no remit to allow any unconsented construction work in a coastal marine area, no matter what it is.
"Reason doesn't come into Resource Management Act - you are either allowed to do it or you are not. This means the council has made a decision outside legal parameters; basically they have just allowed the Dukes to carry on."
Coleman says the ultimate goal of the objectors, who include members of the Herne Bay Residents Association, is to have the boatshed returned to as close as possible to how it was before the helipad construction started.
But he worries that the further the building work progresses, including the work the council has agreed to, the more the new shape and size of the structure could be considered a fait accompli.
And that could mean a court is less likely to make Duke return the boatshed to its pre-helipad state, Coleman says.
"The Act provides for unconsented work to be restored to the environment it was in before the activity began. That's what we want."
A council spokesperson confirmed the Dukes had been given permission to make the site secure and watertight, and said those activities were allowed under the RMA, regardless of the situation with the resource consents.
He said no other construction work would be carried out and the council was monitoring the site closely.
"None of the permitted tasks relate to progressing construction of the previously consented boatshed."
Coleman says as soon as he heard work would be happening on the site, he applied to the Environment Court for an urgent enforcement order which would stop all construction. Now the Dukes' lawyers have been given until Thursday to respond.
An Environment Court hearing on resource consent breaches, where the council and Kawau Island Action both happen to be acting against the Dukes, is due later this month.
The National Business Review's annual Rich List estimates Duke, head of Briscoe Group, to be worth $750 million. His lawyer Richard Brabant did not reply to BusinessDesk's phone call or text message.