Bradshaw, the chief executive of MTF in Dunedin, cited business commitments as the reason for his resignation.
He had been a member of the board since 2007 and his term was due to end in the middle of this year.
Bradshaw (61), an independent appointment by the New Zealand Rugby Union, said with a whole lot of regulations coming into his business this year he had less time to devote to other interests.
"I feel they should find someone who can devote more time and focus to the board and have found with the amount of work on and what will come this year that I've had to discharge some responsibilities," Bradshaw said.
He had enjoyed his time on the board and working with Highlanders management.
He felt the Highlanders board had done well but was always doing it tough in the smallest market in the country.
"By percentages, we get a far greater share of support than a place like Auckland. But I think it is a wider issue than just the Highlanders."
Rugby, and every sport really, has to find the answer to the same question: "How can we compete with everything else on offer?"
He said Highlanders fans were very supportive and demanding but that should be viewed as a positive for the franchise.
Board chairman Ross Laidlaw, who was elected to the board as chairman in October, said the board had gone through some major changes in terms of personnel in the past year but denied there had been a clean-out.
"Sometimes, after four or five years as a director on a board, you've got to look at yourself and wonder if you have got anything left to contribute," he said.
Malcolm Farry, an independent member, resigned after five years on the board in May last year due to a conflict of interest with the building of the new Forsyth Barr stadium, and Steve Thompson stood aside in the middle of last year due to business commitments.
Southland representative Gary Muir was replaced by Invercargill accountant Murray Acker last year and Stuart McLauchlan stood down as chairman in October, because of business commitments, after being in the role for more than five years.
In July last year, the New Zealand Rugby Union had agreed to underwrite the Highlanders for the next two years but said it also wanted to appoint its own board member, and NZRU board member Mike Eagle was appointed in October last year.
The Dunedin City Council was also given a place on the board and Academy of Sport South Island chief executive Kereyn Smith was appointed as the council representative last month.
With Bradshaw's appointment running out at the middle of the year, Laidlaw said the NZRU would not be making an appointment until then, which should give it plenty of time to find the right person.
Board resignations
Highlanders board members left in past year: Stuart McLauchlan (chairman), Angus Bradshaw, Malcolm Farry, Gary Muir, Steve Thompson.
New board members in past year: Mike Eagle, Ross Laidlaw (chairman), Murray Acker, Kereyn Smith.
Other board members: Michael de Buyzer, Adrian Read.
