Former NZ finance minister Steven Joyce made the revelation in his newly released book, On The Record.
Then a party operative, Mr Joyce writes he travelled to Australia in September 2003 to "both observe the Liberal Party's federal election campaign and meet our prospective campaign director ... a guy called Scott Morrison".
In an interview with AAP, Mr Joyce said Mr Morrison verbally agreed the deal.
"It all looked very, very promising. We thought 'Well, this guy will be able to do the job'," he said.
"Then we came home to New Zealand and shortly thereafter he had to turn us down because he'd been shoulder tapped by the Liberal Party and encouraged to stay in Australia because there was a seat coming up.
"It was a verbal agreement. We didn't get to signing on the dotted line."
Mr Morrison was no stranger to NZ, working as an advisor to tourism minister Murray McCully and a senior tourism bureaucrat in Wellington between 1998 and 2000.
He had experience running Australian campaigns, overseeing Liberal campaigns in for the 2001 federal election and for NSW in 2003.
"Jen and I were familiar with NZ and had enjoyed our time together there,"Â Mr Morrison told AAP.
"The offer to come and work on the Nationals 2005 campaign was therefore of interest at that time, and I discussed it with (National leader) Don Brash.
"Other opportunities presented themselves in Australia and we chose to remain. I did however stay in contact with my Kiwi friends, including Steven and followed their progress."
Mr Morrison went on to become managing director of Tourism Australia, the MP for Cook in 2007, and ultimately, prime minister from 2018 to 2022.
Without a campaign director for the 2005 Kiwi election, Mr Joyce took up the role himself successfully - increasing National's vote from 21 per cent in the 2002 poll to 38 per cent, and almost defeating Helen Clark's government.
Mr Joyce went on to lead four more campaigns for National as it took power in 2008, entering parliament as an MP at that election, and serving as infrastructure and finance minister.
"It was certainly a sliding doors moment for me," he said.
"(Morrison) would have to judge whether there was one for him, but he certainly got to be Prime Minister of Australia at the end of it, so that worked out well."
Mr Joyce was tapped by Mr Morrison's government to review Australia's vocational training system, and then by the Liberals to run a review into their 2019 election.
Mr Joyce has stepped back from the political fray but remains a keen observer of Kiwi politics.
Two weeks ago, he told TVNZ this year's election was "wider open" than the five campaigns he ran from 2005 to 2017.
He now senses New Zealanders are leaning towards returning power to the National party.
"It's moving more towards a change," he said.
"The public are getting to a point where they are keen to make a change and that's because the incumbent government, probably more than anything else, hasn't been able to execute policy decisions very well.
"And the economy is quite clearly in need of a reset and the question is, 'Can the current government achieve that reset?'
"The public wants a reset and they'll be looking for somebody who can do that reset for them."
