Non-road mobile machinery (NRMM) in Hong Kong emits considerable emissions and has been a major contributor of environmental regression and harm to the health of citizens. The severe problem caused by considerable NRMM diesel emissions has drawn the attention of the Hong Kong Government. However, the existing measures for reducing NRMM emission mainly target new NRMM while old in-service ones are being left largely unregulated. After a long time of use, in-service NRMM becomes inefficient through deterioration, which results in higher levels of diesel emissions, which are extremely harmful to public health. Transforming in-use dirty NRMM which are exempted from regulations on being greener and more fuel-efficient is an extremely urgent direction for the Hong Kong Government to pursue. Retrofitting in-service high-emitting NRMM can remarkably reduce emissions by using emission reduction technologies, improve the air quality, and achieve notable health benefits for people living or working in or near construction sites. Technologies for reducing emissions from NRMM are crucial for making NRMM greener and cleaner. Thus, this study reviews the technologies widely used for retrofitting NRMM to reduce emissions, to provide some significant insights and recommendations in the case of Hong Kong for initiating retrofit programmes and promote the application of emission reduction technologies.