Absolute photoionization cross-section calculations are presented for Se$^+$ using large-scale close-coupling calculations within the Breit-Pauli and Dirac-Coulomb R-matrix approximations. The results from our theoretical work are compared with recent measurements made at the Advanced Light Source (ALS) radiation facility in Berkeley, California, USA. We report on results for the photon energy range 18.0 eV -- 31.0 eV, which spans the ionization thresholds of the $\rm ^4S^o_{3/2}$ ground state and the low-lying $\rm ^2D^o_{5/2,3/2}$ and $\rm ^2P^o_{3/2,1/2}$ metastable states. Metastable fractions are inferred from our present work. Resonance energies and quantum defects of the prominent Rydberg resonances series identified in the spectra are compared for the $\rm 4p \rightarrow nd$ transitions with the recent ALS experimental measurements made on this complex trans-iron element.