Thermal bridging is known to affect the thermal performance of lightweight steel-framed external walls. Various manual calculation methods have been developed to estimate this impact, including the parallel path, NZS 4214 and modified zone methods, with the NZS 4214 method recently adopted for non-residential buildings by the Australian National Construction Code 2019 via its explicit reference in AS/NZS 4859.2. These methods have been validated against different wall assemblies from those currently used in the Australian context, therefore this paper aims to examine the validity of thermal bridging calculation methods in this context and evaluate the impact of mitigation methods on the overall thermal performance. We developed a set of 34 realistic external cold-formed steel-framed wall configurations based on available market products and construction practices and compared the R-values from manual calculation methods with outputs from the 2D simulation THERM. Results have shown that the NZS 4214 method as recently adopted by the Australian National Construction Code 2019 for non-residential buildings has good applicability for wall assemblies with lower R-values (R = 1.8–2.3 m2K/W). However, it was shown to have larger differences for calculations of walls with higher R-values (R > 2.5 m2K/W). The isothermal planes method can produce various results for the same building assembly, depending on which planes are assumed to be isothermal. This study has demonstrated that superior accuracy may be obtained by varying the definition of such planes depending on the R-value of the assembly. When considering designs to mitigate thermal bridging effects and improve overall wall performances, outside frame insulation and high resistance claddings have been shown to provide the best results (decreasing variation in temperature across the surface by 75%).