Hadwiger's conjecture asserts that every Kt‐minor‐free graph has a proper (t−1)‐colouring. We relax the conclusion in Hadwiger's conjecture via improper colourings. We prove that every Kt‐minor‐free graph is (2t−2)‐colourable with monochromatic components of order at most ⌈0false12(t−2)⌉. This result has no more colours and much smaller monochromatic components than all previous results in this direction. We then prove that every Kt‐minor‐free graph is (t−1)‐colourable with monochromatic degree at most t−2. This is the best known degree bound for such a result. Both these theorems are based on a decomposition method of independent interest. We give analogous results for Ks,t‐minor‐free graphs, which lead to improved bounds on generalised colouring numbers for these classes. Finally, we prove that graphs containing no Kt‐immersion are 2‐colourable with bounded monochromatic degree.