  In this paper we compute the electroweak corrections to the charged Higgs
boson decay into a $W$ boson and a neutral Higgs boson in the CP-conserving
NMSSM. We calculate the process in a general $R_\xi$ gauge and investigate the
dependence of the loop-corrected decay width on the gauge parameter $\xi$. The
gauge dependence arises from the mixing of different loop orders. Phenomenology
requires the inclusion of mass and mixing corrections to the external Higgs
bosons in order to match the experimentally measured mass values. As a result,
we move away from a strict one-loop calculation and consequently mix orders in
perturbation theory. Moreover, determination of the loop-corrected masses in an
iterative procedure also results in the mixing of different loop orders. Gauge
dependence then arises from the mismatch with tree-level Goldstone boson
couplings that are applied in the loop calculation, and from the gauge
dependence of the loop-corrected masses themselves. We find that the gauge
dependence is significant.
