The gallery extension involves construction at the rear of the existing gallery building including removal of trees, some additions and changes to the roads and footpaths, service installations and landscaping and planting.
An initial archaeology assessment has pulled together background on the park's history and recommended what needs to happen when breaking ground.
All construction earthworks will be monitored by an archaeologist who will be at the ready with processes in place if any artefacts are unearthed.
Several archaeological studies in the park over recent years for construction work, including the footpath to the cenotaph in 2002, have revealed the area contains nationally important evidence of the Rutland Stockade (constructed 1846-47).
It was one of the largest of at least 32 redoubts, stockades and blockhouses built by the colonial government or settlers in the decade following Te Tiriti o Waitangi/the Treaty of Waitangi.
Although the hill was once a pā, little is known about Pukenamu's history. The site was a defensive position for the Whanganui River people, featuring in stories of battles between Māori before Pakeha arrived.
Modelled on the pā hill-fort, the Rutland Stockade was on the high point and the hill's eastern end, with a soldiers' settlement on the northern part sheltered from prevailing winds. Early photographs and maps show several houses or huts were close to the gallery extension.
The stockade blockhouses were used as a prison for a while, and after the last troops left in the 1870s squatters took over and the area became known as 'The Rookery'.
The Wanganui Girls' School opened in 1880, behind where the Davis Library now stands, with part of the school's grounds levelled by prisoners. After the school partially burnt down it was refurbished and expanded to become Queen's Park School.
In 1882 the Wanganui Borough Council started a programme of beautification on the hill, which created much of the park's landscape today and by 1887 what was left of the stockade was deemed derelict and finally demolished.