Left: [OIII]/Hβ vs. [NII]/Hα diagnostic diagram for the 954 out of the 3326 galaxies with both line flux ratio measurements from the MPA/JHU catalogue. As in Fig. 1, the 135 sources above our IR colour cut are shown as red filled triangles. The dashed line is the demarcation line between normal star-forming galaxies and AGN from Stasinska et al. (2006). Middle: same as left, but showing the 126 AGN that were previously identified in the MRBGD optical samples split into 111 type 1 (BL; green filled triangles) and 16 type 2 (NL; blue filled triangles). One-third of this MRBGD sample are located in the region for HII galaxies. The 9 candidate AGN, selected using the IR diagnostic, which are not in the samples of MRBGD are shown as red filled triangles. Right: same as left, but showing the 83 BCDs (cyan filled triangles) and the two low-metallicity and heavily obscured BCDs that were originally identified by Griffith et al. (2011) (magenta filled circles). The BCD MRK 709 S (yellow filled circle), one of the most metal-poor BCDs with X-ray and radio emission indicative of AGN activity, is also located in the region for HII galaxies.