But Robertson said even that was not enough.
"To meet the expectations of the role, MPC members will need a significant amount of experience in exercising sound judgment under high levels of uncertainty."
Robertson said there would be only a small number of people who fit this category and, to attract them, they would need to be paid more.
"MPC members will not be able to hold some potentially lucrative financial sector jobs due to potential conflicts of interest," Robertson said.
Asked how much the members would be paid, a spokesman for Hipkins said specific amounts were "sensitive information" and, as such, would not be disclosed.
"This is on the basis that it could weaken the Government's ability to negotiate fee levels by creating an environment where the exceptional fee becomes the norm."
The letter also said the most comparable role within the state sector would be a member of the Commerce Commission, who earns a salary equivalent to a daily fee of $1565.
It also noted MPC members at the Bank of England receive around $1900 in New Zealand dollars.
"Reserve Bank of New Zealand external MPC members will require similar economic and analytical skills, although their role is likely to be less public facing," Robertson said in the letter.
He said the expectation is that external MPC members would dedicate around 50 days per year to the role.
A spokesman for Hipkins said in 2017/18, the Government approved 43 "exceptional fee" proposals.
That number was 90 in 2016/17 and 42 in 2016/15.
In late March, Robertson announced the appointment of Caroline Saunders, Bob Buckle and Peter Harris as the external members of the MPC.