Functionalisation of nitrogen–nitrogen bonds of antisite defective boron nitride nanotubes (BNNTs), including exchange antisite defect which is produced by the rotation of BN bond, and substitutional antisite defect which is formed by substitution of an N with B, is investigated through their interaction with a $\rm{B^{−}_{6}}$ cluster. The smaller defect formation energies for the substitutional antisite defects indicate that the substitution of an N atom with B atom is easier than rotation of a BN bond. The formation of antisite defects at the edge or near the edges is more favourable than that in the middle of the tubes. When complexation between double ring $\rm{B^{−}_{6}}$ and nitrogen–nitrogen bonds of antisite defective BNNTs occurs, two-fold coordination, double ring configuration of boron cluster and N–N bond cleavage are seen. In the most stable complex, the $\rm{B^{−}_{6}}$ pulls apart the B–N bond and becomes an integral part of the tube by expanding the hexagonal BN ring, while in the other BNNT-B6 clusters, double ring $\rm{B^{−}_{6}}$ acts as a bridge at the top of the decagon. Functionalisation of N–N bonds at the edge or near the edges is more favourable than that in the middle of tubes.