Pop-up messages are used to minimize harms due to excessive gambling in many countries. In New Zealand, electronic gaming machines (EGMs) are required by law to include a pop-up message feature since 2009. Pop-up messages interrupt gamblers at random time after 30 minutes of continuous gambling. They provide gamblers with information about their current gambling session, the amount of money they had spent, and net wins or losses. After 15 seconds, gamblers can choose to continue or stop gambling. To date, New Zealand is the only country in which EGMs in every gambling venue must include pop-up messages. This study investigated the effects of mandatory pop-up messages on amount of money spent on gambling. It also explored how EGM gamblers interacted with pop-up messages.