The use of compressed natural gas (CNG) as a fuel for heavy transport engines is still in the development stage, however, experience has shown promising results in certain applications. While most of the recent work done in New Zealand on CNG use has been centered around non-turbocharged, high speed, spark ignition (SI) conversions, there is still likely to be a role for dual fuel engines in some applications.This paper presents a review of New Zealand experiences with the use of CNG as a fuel in heavy transport diesel engines. Both dual fuel and 100% CNG fuelling (dedicated spark ignition conversions) are discussed. The advantages and disadvantages of each are reviewed in terms of vehicle performance, fuel costs and operational experiences. Brief reviews of experimental results and discussions of technical aspects of conversions, experience in fleet use, economic and operational aspects, current research and computer modelling of fleet performance are also included.