We study the influence of the tetragonalization occurring during the martensitic phase transition on the exchange interactions in Ni₂MnGa Heusler alloy using first-principles calculations in conjunction with the frozen-magnon approximation. We show that the tetragonalization alters only the exchange constants characterizing the Mn–Mn interactions. Calculated Curie temperatures within the random-phase approximation are found to agree with experimental data. Moreover, we study the temperature dependence of the magnetization and the small deviation from the experimental data exactly at the temperature of the phase transition is discussed. Obtained results agree with previous theoretical results using Liechtenstein's formula to calculate the exchange constants and the Monte Carlo simulation technique to estimate the Curie temperature.