<p>Negatively charged nitrogen vacancy (NV⁻) centres in diamond crystals are promising colour centres for high-sensitivity quantum sensors. A long dephasing time (<i>T</i>₂^* &gt; 10 μs) is essential for achieving increased sensitivity and higher uniformity of <i>T</i>₂^* in millimetre-scale diamond is strongly desired for femto-tesla weak magnetic field detection. High uniformity of <i>T</i>₂^* for NV⁻ centres is achieved herein. The median value of <i>T</i>₂^*, &lt;<i>T</i>₂^*&gt;, in the ¹²C-enriched high-pressure, high-temperature (HPHT) grown diamond with a nitrogen concentration of 1.3 ± 0.4 ppm is 4.5 μs. The variance of <i>T</i>₂^* is only 10% over a millimetre-scale region (1.1 × 1.1 mm²) within the 0.4 mm thick {111} growth sector. &lt;<i>T</i>₂^*&gt; is ~2/3 times the value limited by the dipole-dipole interaction from the electron-spin bath of nitrogen impurities, suggesting that the residual strain gradient in the HPHT diamond crystal partially limits <i>T</i>₂^*. Reducing the strain gradient in diamond crystals provide a pathway to achievement of high sensitivity magnetometry using NV quantum sensing.</p>