He also wanted to spend more time with his family, including a baby son, in Napier and work on building up support for Labour in that electorate.
But insiders said his departure was hastened by conflict within Mr Shearer's office.
Mr Nash and, to a lesser extent, John Pagani - another of Mr Shearer's advisers - are understood to have disagreed with his chief press secretary, Fran Mold, about the extent to which Mr Shearer should lead attacks on the Government rather than refuse to be drawn into oppositional politics.
Mr Nash is believed to have been keen for Mr Shearer to focus on building up his "non-politician" image, focusing on being optimistic rather than engaging with National.
But Mold and others in Mr Shearer's leadership team believed that was being taken too far and starving Mr Shearer of the media coverage he needed.
Mr Shearer last week led the charge on the Crafar farms sale, but the previous strategy of keeping him away from subjects on which Labour had a negative line gave rise to speculation about the leadership after his deputy, Mr Robertson, was left to lead Labour's reaction on major issues such as Nick Smith's resignation as minister over troubles in ACC.
Mr Shearer said he did lead reaction on many issues but also recognised that his other MPs were "very able" and should be allowed to contribute.
The changes are a disruption Mr Shearer could do without as he tries to make an impact in the polls. Although it is not yet serious, he is already having to contend with speculation about his leadership and the ambition of his deputy.
Most of it that is driven by right-wing blogs, such as Cam Slater's Whale Oil blog in which he has wondered whether Mr Cameron's appointment was driven by Mr Robertson filling key positions with his loyalists.
Mr Robertson has dismissed the speculation, but it has spread to others, including the left-wing blog the Standard.
Contributor Irish Bill observed that it was no secret Mr Robertson wanted the job and, although he hoped he was wrong, "it's starting to feel like a leadership challenge is inevitable".
Mr Shearer said it was a long process to get Labour back to being "match fit" and there was an acceptance of that. He said he was not aware of tension within his office.
Labour's general secretary, Chris Flatt, is also understood to be leaving after almost three years for a job with the Dairy Workers' Union. He would not confirm the change last week.
