Cameron made the remarks during an event to promote the New Zealand film industry in Wellington.
Last year, Cameron said he plans to release the first "Avatar" sequel in time for Christmas 2016 and the following sequels in late 2017 and late 2018.
During Wednesday's event, Cameron said the delay comes as he is working on scripts of the three sequels at the same time.
"Avatar," a science-fiction epic, is the highest-grossing films of all time. Its box-office takings worldwide amounted to US$2.78 billion.