Janet Frame made it pretty clear that she was leaving a large amount of unpublished work for 'posterity'.
Frame mentioned her unpublished poetry and stories and novels often (in radio and press interviews, in letters and conversations quoted in Michael King's biography, in her autobiography); she even publicly named some of her unpublished pieces and novels (eg Towards Another Summer, 'Gorse is Not People'). Many anecdotes from the literary world tell of conversations Frame had with friends and colleagues in which she mentioned her unpublished work and her desire especially to publish more of her poems and other writings. For instance Bill Manhire is quoted in a recent NZ Listener interview as saying he had discovered a letter to him from Janet Frame from the mid 1980s in which she wanted to discuss publishing a volume of poems and stories. (For whatever reason, this project never eventuated.) And Greg O'Brien has spoken of Janet Frame telling him about "the novel I wrote in France" (In the Memorial Room, soon to be published.)
In her lifetime Frame explained many of the reasons why she kept some of her work back, and she left some explicit instructions and some clues about what should be done with it after her death.
Her editors have outlined these reasons in the introductions, prefaces and afterwords to the publications pictured above. One volume The Goose Bath (the first of the posthumous publications) has all three: a preface, an introduction, and an afterword!
Frame's posthumous editors (and I am of course one of these) have never attempted to 'justify' posthumous publication of Frame, as in every case we have been confident that Janet Frame's work was capable of being taken at face value and worthy of being judged on its own literary merits rather than being subsumed as mere biographical curiosity. We have where appropriate laid out Frame's own explanations (for instance, the reluctance of some publishers to print poetry or stories when novels 'sell better') and we have also made educated guesses as to other possible factors.
But in the end, the only watchword and justification for publication is the quality of the work.