Nelson City Council is taking legal action after a ramshackle structure was constructed without permission in the city's harbour.
The shack, sitting on four stilts, has been built close to the most northern of the Boulder Bank's historic baches and poses a navigational hazard.
The construction first came to the attention of the city council last month after concerns from Nelson Rowing Club, which trains there.
The builder of the shack has so far refused to comply with council demands to remove it.
He has continued to expand and improve the building, despite having been issued a two-week abatement notice.
The council says it is now seeking an enforcement order from the Environment Court to have the structure taken down.
