The comments were made at a hui at Terenga Paraoa Marae in Whangarei last Thursday.
The controversial legislation stripped iwi rights to seek customary title to marine and coastal areas.
David Cunliffe said the Labour government at the time should have consulted more with Maori, and should not have removed their right to a legal process.
He said apologising to Maori for the Act was something they could revisit if elected, and he also thanked those who were loyal to Labour during that time including the late kaumatua Tapihana Shelford and Parekura Horomia.
But Mr Cunliffe stressed it was his personal view, not Labour policy, shared on the marae.