Going by (the abstract of) a paper i came across [1], it's possible-to-likely that this was the original meaning of 'mana', and that the use to refer to some kind of abstract supernatural charge is either a derivation or a misunderstanding.
The abstract begins:
Comparative data are assembled to suggest that in Proto-Oceanic, mana was canonically a stative verb meaning 'be efficaceous, be successful, be realized, "work"'. Where mana was used as a noun, it was (and in most daughter languages is) not a substantive but an abstract verbal noun: 'efficacy', 'success', 'potency'.
The abstract begins:
Comparative data are assembled to suggest that in Proto-Oceanic, mana was canonically a stative verb meaning 'be efficaceous, be successful, be realized, "work"'. Where mana was used as a noun, it was (and in most daughter languages is) not a substantive but an abstract verbal noun: 'efficacy', 'success', 'potency'.
[1] http://www.jstor.org/discover/10.2307/3629696?uid=3738032&ui...