The two tribes' settlements, both worth $170 million, were settled in the mid 1990s with clauses that enable them to seek a top-up if the total sum of treaty settlements surpass $1 billion in 1994 terms.
Dr Cullen said the $1 billion envelope - originally set for treaty settlements - was abandoned in 1996 when it was realised the real final figure would go much higher.
Dr Cullen today acknowledged the inflation-adjusted $1 billion figure was likely to be passed in the next "two to three" years triggering Ngai Tahu's and Tainui's rights to return to the negotiating table.
"With a range of settlements under way we're going to pass that $1 billion threshold fairly soon, adjusted by CPI," he told Parliament's Maori Affairs select committee.
"We will reach that point and from that stage on, in effect, any quantum elements in redress will trigger ... a 17 percent increase to Ngai Tahu and Tainui." Dr Cullen said Tainui and Ngai Tahu - among the first to settle - were the only tribes with relativity clauses and subsequent settlements were full and final.
They would not be reopened. Dr Cullen said several treaty deals were currently in train, including the central North Islands "Treelords" deal, worth about $400 million. He said the range of deals under negotiation was putting the Office of Treaty settlements under stress.
Momentum was continuing to build as other tribes accelerated their own claims as they saw those in negotiations making solid progress. Despite the fact that negotiating resources were being stretched, the trend was extremely positive.
Dr Cullen said he expected four new pieces of settlement legislation before Parliament by the time of this year's election.
He was confident the Government could meet its target of settling all historic claims by 2020.
He said an affiliate Te Arawa settlement bill would hopefully go through its first reading next Tuesday. He also hoped to sign the central North Island deed of settlement on Wednesday and have the first reading of the settlement bill that afternoon.
The other bills that he expected to be before Parliament this year were one settling Tainui's claim on the Waikato River and another settling Taranaki Whanui's claim. He expected the Te Roroa Claims Settlement Bill to be passed before the election.
