A tin shack illegally constructed on the Boulder Bank in Nelson last year has become a houseboat.

The single room built of pallets appears to have been lifted off its four supporting poles and floated down the harbour some 100m before being anchored at the Boulder Bank opposite the turnoff for Marybank Rd in Atawhai. It now sits on top of a boat, and has drums lashed to it for flotation.

During the shack's construction below the high tide line, the Nelson City Council, Port Nelson and the Department of Conservation all issued statements requesting its removal.

The structure was identified as a risk to navigators while in its original position last year. Harbourmaster Dave Duncan said he could not yet confirm whether it still posed a hazard as his staff were unsure exactly where it had ended up.

"Because it's now adrift, it has to comply with a different set of regulations, [although] it never did comply in the first place."

Nelson City Council community relations manager Angela Ricker said the council had obtained an enforcement order from the Environment Court requiring the builder of the unlawful structure to take it away. He has previously ignored a council abatement notice giving him two weeks to pull down the shack.

She said the council was aware that the structure was now floating, and would visit the site where it was formerly erected to see if anything was left behind. Council officers and the harbourmaster also plan to visit the site to see whether the houseboat poses a navigation safety hazard.

"Even though the structure now floats, if it is being used to unlawfully occupy the coastal marine area the council will take the appropriate action to get it removed," Ricker said.

DOC is responsible for administering the Boulder Bank, issuing a statement in September saying it supported the council's efforts.