Organized defect clusters in non-stoichiometric LiNbO₃ are known to dominate macroscale ferroelectric properties; yet the detailed nature of these defects is currently unknown. Here, the relative stabilities of various defect cluster arrangements of lithium vacancies around a niobium antisite in LiNbO₃ are determined using density functional theory combined with thermodynamic calculations. Their effects on the ferroelectricity of the system are also discussed. It is found that at room temperature the non-uniaxial dipole moments associated with the defect clusters could affect the properties of the system locally. The diffusion mechanism is predicted to be through first nearest neighbor jumps on the Li sublattice. The diffusivity of the lithium vacancy is found to be extremely low at room temperature, which indicates that the defect complexes should be rather stable.