MENZ Issues

IRD again

What I dont get is if you add my income and my ex income together and divide by 50%…the inland revenue seem to think it takes near on $600 a week to raise two teens…so I have to pay her $300 a week, when she
earns almost as much as me. The way I see it she will not need to put her hand in her pocket for the kids at all. My $300 a week I give the IRD will cover all their costs. I so dont get this