Dr Brash also said Act and National had confirmed the same agreement in Epsom as they had under Mr Hide - that National would campaign primarily for the party vote.
He said Act would perhaps be stronger if its candidate won Epsom with no assistance from National, but it was better to give voters a sure way to elect a stable Government "even if it slightly weakens Act's ability to drive a hard bargain in post-election talks".
"We have a duty to ensure stability."
Former Auckland city mayor and National cabinet minister John Banks is standing for Act in Epsom, replacing Mr Hide.
Dr Brash, the Reserve Bank Governor for 14 years, also criticised the Tea Party Republicans in the United States, saying they had "played chicken with the world's largest economy".
While he agreed that spending had to be brought under control, "in our judgment the Tea Party people were so one-eyed that they were not willing to discuss a constructive way out of what in the US is a very serious fiscal dilemma".
"The economy needs confidence. In the last few weeks, businesses all over America have put away their cheque books as events in Washington have made people scared to invest, to hire and to buy.
"The damage to the American economy from the debt debacle may turn out to cost the US Government more in lost revenue than the spending savings the Republicans achieved."
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Labour list MP David Parker has been confirmed as Labour's candidate for the Epsom electorate.
Mr Parker has been a Dunedin list MP.
He will face Mr Banks, a former National cabinet minister and Auckland City mayor.