The operator says several services have been withdrawn and a limited tram service is currently running, meaning passengers should expect extended wait times.
NET said the decision had been made on safety grounds because some trams were struggling to remain at a safe and cool temperature during the hot weather.
Services which cannot provide a cool enough environment for passengers are being taken out of operation so the trams can be cooled at the depot and maintenance teams can access vehicles to fix any issues.
NET apologised for any inconvenience caused.
