If I plan or have installed Solar PV system for self-consumption (SELCO) and have taken the necessary steps to ensure no reverse flow or energy exported to the grid, do I need to apply for NEM?

No, but for PV system with capacity above 72 kWp, generation license (private) from ST is required and subject to complying with the relevant rules under the Electricity Supply Act (ESA). For more information please refer to the Guidelines on the Connection of Solar Photovoltaic Installation for Self-Consumption issued by ST.