Thermodynamic properties as well as low-energy magnon excitations of S = 1 honeycomb-layered Na3Ni2SbO6 have been investigated by high-resolution dilatometry, static magnetisation, and highfrequency electron spin resonance studies in magnetic fields up to 16 T. At TN = 16.5 K, there is a tricritical point separating two distinct antiferromagnetic phases AF1 and AF2 from the paramagnetic regime. In addition, our data imply short-range antiferromagnetic correlations at least up to ∼ 5·TN. Well below TN, the magnetic field BC1≈ 9.5 T is needed to stabilize AF2 against AF1. The thermal expansion and magnetostriction anomalies at TN and BC1 imply significant magnetoelastic coupling, both of which associated with a sign change of ∂L/∂B. The transition at BC1 is associated with softening of the antiferromagnetic resonance modes observed in the electron spin resonance spectra. The anisotropy gap ∆ = 360 GHz implies considerable uniaxial anisotropy. We conclude the crucial role of axial anisotropy favoring the AF1 spin structure over the AF2 one. While the magnetostriction data disprove a simple spin-flop scenario at BC1, the nature of a second transition at BC2 ≈ 13 T remains unclear. Both the sign of the magnetostriction and Grüneisen analysis suggest the short-range correlations at high temperatures to be of AF2-type.