Mathematica fast 2D binning algorithm

Question

I am having some trouble developing a suitably fast binning algorithm in Mathematica. I have a large (~100k elements) data set of the form T={{x1,y1,z1},{x2,y2,z2},....} and I want to bin it into a 2D array of around 100x100 bins, with the bin value being given by the sum of the Z values that fall into each bin.

Currently I am iterating through each element of the table, using Select to pick out which bin it is supposed to be in based on lists of bin boundaries, and adding the z value to a list of values occupying that bin. At the end I map Total onto the list of bins, summing their contents (I do this because I sometimes want to do other things, like maximize).

I have tried using Gather and other such functions to do this but the above method was ridiculously faster, though perhaps I am using Gather poorly. Anyway It still takes a few minutes to do the sorting by my method and I feel like Mathematica can do better. Does anyone have a nice efficient algorithm handy?