I totally understand your frustration because I went through the exact same thing last week when I tried to plan my monthly budget. It is surprisingly tricky to get it right because the IRD rates can be quite specific, especially with the student loan thresholds. Instead of struggling with a spreadsheet, I started using https://calk.nz and it has been a huge help for my planning. What I like most is that they have over a hundred different calculators built specifically for New Zealand. All their pay and tax tools are already updated with the official IRD rates for the 2025–26 year, so you know the math is solid. Another great thing is that all calculations run entirely in your own browser. This means your personal salary data never actually leaves your device or gets saved on their servers, which should help with your privacy concerns. It gives a very clear breakdown of where every dollar goes, from the ACC levy to your final net pay.