Little said McCarten's work was not campaigning but "outreach" for Little such as organising events and meetings when Little was in Auckland.
He denied he was trying to use taxpayer funds for campaign-related work, saying party work would be done by party workers in the same office rather than McCarten and other Parliamentary-funded staff.
"I know the level of scrutiny that is applied to Parliamentary Services funding. We are always looking to make sure we are well within the rules, not just the written word but the spirit of it as well. That will continue to apply in every appointment I make, every activity I do."
He said there was nothing unusual about the arrangement.
Little got defensive after further questions, saying the media could "pick apart employment arrangements" but he was more concerned with the issues facing Aucklanders such as housing and burglaries.
In the National Party some staff have their salaries partly paid by the party to account for a portion of party related work.
McCarten's shift risks leaving Little without a chief of staff or a chief press secretary - two key roles.
Little's former chief press secretary Sarah Stuart also left in May and is yet to be replaced.
Two further long-serving press secretaries have now also resigned from Little's office to take up jobs outside politics
Little denied an apparent exodus a year before the election was a sign of expected defeat.
The departures of the two will leave Labour's press team at half strength.
Little said changes in the office staffing were "pretty standard" and "not at all" a sign the staff did not expect Labour to win the 2017 election.
A recruitment process for the chief press secretary was still underway.
He said that was delayed because the party was in the midst of deciding on its election year staffing, including the establishment of the new Auckland office.