The board had adopted an ambitious strategy, he said.
"They don't just want to be an adjunct of the Government. Act wants to be the third force in politics."
The former Auckland Central, Wellington Central and list MP said he had urged Jamie Whyte to seek the leadership - which he won three weeks ago.
"Then he rang me up and said would I be his campaign director, and having urged him to take the job, I couldn't think of an excuse fast enough."
Mr Prebble said the campaign would likely revive the party's flat-tax policy as well as coming up with fresh policies.
"I expect to see Act talking more about the policies it first talked about."
Dr Whyte had said he wanted to take the party back to its principles when it was the Association of Consumers and Taxpayers.
"The best known of those policies is Act's belief that the fairest sort of tax system is a flat tax where everyone pays the same rate."
The party also had strong policies on social welfare reform, and savings and superannuation.
Act's annual conference will be held at the Villa Maria Estate in Mangere, Auckland, next weekend, and has been dubbed by the party as "the Thriller at the Villa".
Current leader John Banks will hand over to Dr Whyte, a philosopher, writer and former management consultant.
Mr Banks is also MP for Epsom, a seat Act has held since former leader Rodney Hide won it in 2005 - without endorsement from National.
Mr Prebble believed Mr Hide and Mr Banks had earned respect in Epsom for being good local MPs and that would be a good base for Epsom candidate David Seymour to build on.
"We are determined to try to hold that seat but we are going on a wider strategy," Mr Prebble said.
"Jamie Whyte, by standing on the list only, is making a statement that we want the list vote [party vote] too."
Mr Prebble said he was expecting 200 or so members at the conference.
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