There's been a fire at a hotel under construction in Christchurch.

It broke out in the old Forsyth Barr building around 3am, which is being turned into a new Crowne Plaza hotel.

The Fire Service said it started in building materials on about the sixth floor, and was difficult for crews to get to because the site was locked up.

Firefighters eventually left at around 5 this morning.

The fire is not being treated as suspicious and the building has been handed back to the construction company.