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Bound states in the continuum (BICs) are widely exploited in all-dielectric metasurfaces to significantly enhance the Q-factor and second harmonic generation (SHG). However, a higher SHG conversion efficiency is overshadowed by the narrow transparency window of all-dielectric materials and strict requirement of polarization-matching. Herein, an augmented SHG assisted is demonstrated by symmetry-protected quasi-BICs from z-cut wurtzite GaN metasurfaces, whose wide transparent range suppresses self-absorption of second harmonic. Strong resonances emerge under both X- and Y-polarized excitations, where the multipole characters of each eigenmode are quantitatively identified via decomposition simulations. The local field enhancement enabled by the extreme energy confinement of quasi-BICs contributes to an efficient SHG, recording a conversion efficiency up to 2 × 10−7 at moderately low input peak intensity (≈1 GW cm−2), orders of magnitude larger than bulk GaN. With proper design, the overlapping X- and Y-polarized resonances can support SHG from excitation with arbitrary polarizations. The alleviation of polarization-matching remarkably enhances the SHG power by 66.2% under unpolarized illumination condition. The results of intense SHG reveal GaN as a promising candidate for high-performance nanoscale nonlinear photonic devices. The weak polarization sensitivity of the designed structure will chart a novel course for the advancement of next-generation efficient all-dielectric metasurfaces. © 2024 Wiley-VCH GmbH.