Cameron plans to film the highly anticipated sequels at one time and then release the three films over consecutive years.

The first had been due in late 2016, but he said the writing process had proved to be "very involved" and that the initial target date was probably too ambitious.

"There's a layer of complexity in getting the story to work as a saga across three films that you don't get when you're making a stand-alone film," he said in Wellington, New Zealand, where he was helping promote the local film industry with other directors including Peter Jackson.

Cameron said the goal was to have the scripts for the three movies completed by the end of this month. He's leading a team that's writing the screenplays.

"We're writing three simultaneously. And we've done that so that everything tracks throughout the three films. We're not just going to do one and then make up another one and another one after that," he said. "And parallel with that, we're doing all the design. So we've designed all the creatures and the environments."