Sir Roger Douglas, who will be one of ACT's five MPs, is warning the incoming National Government not to expect him to be silent.
Prime Minister-elect John Key reiterated on Monday that Sir Roger, who was a Labour finance minister in the 1980's, will not hold a Cabinet position.
Sir Roger earlier told Nine to Noon it is not certain whether he would remain ruled out, and if he was, there was still room for him as a "sideline commentator."
Sir Roger said his views will be known, and there is "no way" he is going to sit in Parliament over the next three years and be silent.
He said the reforms he made in the 1980s are still in place, unchanged by either a National or Labour-led Government.
National has enough seats to govern with support parties ACT and United Future, after a sweeping victory in the 2008 general election on Saturday.
Bill English, who is due to become Finance Minister in the new Government, said his party is set to start receiving briefings on the state of the economy.
Mr English told Nine to Noon there will be no cabinet position for Sir Roger Douglas, but said the Government would be open to receiving economic advice from a wide range of people.
United Future leader Peter Dunne said ACT should keep in mind that the party holds only five compared to National's 59.
"If it sees its performance on Saturday as a justification for an unleashing of another right-wing binge, then I think that's not what the country wants or needs."