The council revealed plans to plant nikau and phoenix palms and two or three new oak trees to mitigate the effects of losing the protected English oaks.
The idea of using the wood from the removed oak trees to make furniture for the new gallery was also mooted, along with offering some wood to the public. The replacement trees plan comes on the back of suggestions from three submitters, who supported the project but were worried about the removal of the protected trees.
Meanwhile, Mainstreet Wanganui supported the application with the condition that vehicle and pedestrian access and the relationship between the gallery and the city centre be reviewed before the redeveloped gallery opened.
Chairman Peter Robinson suggested a review of the 20-year-old Queen's Park management plan to incorporate the development.
There were fears the new entrance, part of the extension at the current rear of the building, would disconnect the gallery from the city, and Mr Robinson wanted to ensure the current entrance remained the focal point.
"I am concerned that the gallery does not turn its back on the city," he said. "If the existing main entrance in not maintained as a main entrance, it should at least be a prominent and accessible feature not consigned to a tradesman entrance dollied up from time to time for ceremonial occasions."
Bruce Dickson, on behalf of the Wanganui Regional Heritage Trust Board, agreed. "It seems a shame that the historical significance of the entrance is going to be lost, apart from significant occasions."
Retired architect Denis McGowan said the hearing had to weigh up considerable socio-economic and cultural benefits over the breach of a few rules.
"To my mind there is no contest," he said.
The commissioner's decision was reserved and will be released within 15 working days.