One legal expert is challenging Winston Peters' speculation about what happened to the Interislander's Aratere.

The ferry ran aground near Picton in June, and its detention notice has officially lifted this afternoon.

The Aratere has had inspections and tests and will make a graduated return to service - beginning with freight, then trucks, then passengers.

This week, NZ First asked if someone left the ship on auto-pilot while getting a coffee - then couldn't turn it off.

Maritime Union lawyer Troy Stade says the crew absolutely didn't duck out for a coffee.

"And it astounds me that the acting Prime Minister would raise unsubstantiated rumour and conjecture in the middle of an active investigation." 

UPDATE- a leaked document revealed the autopilot button was mistakenly pressed by the crew. To learn more, click here.

LISTEN ABOVE

• Aratere ferry's future uncertain amid pending investigation
• 'Lost the ability to steer the ship': Minister gives update on grounded Interislander
• Grounded Aratere ferry won't be back in service for a while, expert predicts
• 'Crikey, that’s not a denial': Peters to find out more about Aratere grounding