New Act leader Don Brash has begun his election campaign with a "Dear John" letter accusing the Prime Minister of betraying his past principles and pandering to Maori.

In the letter sent to media this morning, Dr Brash claimed John Key had back tracked on National's former policies and accused him of "running New Zealand for our opponents".

He criticised National's decisions to retain the "privileged" position of Maori and to keep student loans interest free.

Mr Key had opposed those policies alongside Dr Brash during National's 2005 general election campaign, the letter said.

"You have spent the last three years building up probably the greatest reserves of prime ministerial popularity in New Zealand history. What's the point of cultivating such influence unless you plan to use it to help our country?

"And if you won't use it now, in this time of crisis, when will you?"

Dr Brash also rails against Mr Key for deciding against re-introducing a youth minimum wage and saying he would resign before raising superannuation eligibility age from 65.

Those policies were unaffordable in a worldwide economic crisis which is seeing New Zealand borrowing $300 million a week, he said.

A spokeswoman for Mr Key said he was unlikely to respond to the letter.

Mr Key has previously labelled Dr Brash an extremist.

He ruled out many of the recommendations of the Dr Brash-chaired 2025 Taskforce into narrowing the wage gap with Australia, saying its economic ideas were too extreme.