For example, does it take into consideration all the credits I’ve ever got over all 3 levels of NECA?
Or is it only of level 2 and level 3?
Many US universities have been moving away from the notion of “GPA” for international applicant since it is more of a practice in US high schools. For instance, if you do NCEA they’ll look at the number of Excellence credits you receive; if you do IB they’ll look at your final IB score out of 45; if you do CIE they’ll look at your IGCSE, AS and A2 grades (out of A* or 9). No conversion is very necessary.
If you really really want to get a numerical measure for your NCEA grades, one way that many students use is the approach that some universities in NZ employe. E has the highest weighting of 4, followed by M which is 3 and A which is 2. This will give you a “GPA” out of 4 or a percentage out of 100%.
Most colleges look at grades in the last four years of high school (Year 10 - 13 in NZ) so make sure your reports across those years are as solid as possible.