Mugabe, 85, said that the new unity government with Prime Minister Morgan Tsvangirai, formed only two weeks ago, was a temporary solution until the parties could agree on a new charter and fresh polls.
"We are an interim arrangement. We are not a permanent inclusive government," Mugabe said. "Ahead of us is a whole constitutional process which requires that we address the issue of the constitution.
"There is already a draft that the three parties agreed on. We shall look at it and when we are all satisfied it shall be put to the people in a referendum", he said.
He said the timeframe agreed on by parties to have a referendum was within 18 to 24 months.
"If the people say yes, then the draft will be allowed to pass through parliament. The timeframe that was agreed on by the parties was that within 18 to 24 months we should have a referendum."
Zimbabwe’s descent into political and economic crisis began nine years ago when Mugabe lost a referendum on a new constitution that would have expanded the powers of the man who has ruled since independence in 1980.
But the crisis deepened after disputed elections last year, sending Zimbabwe into a tailspin that saw politics deadlocked while a humanitarian crisis spiralled out of the control.
Tsvangirai, a long time rival of Mugabe, agreed to form a unity government under intense regional pressure to end the crisis, which has left most of the population without food while a cholera epidemic has killed more than 3 800 people.