There are two kinds of quests that are available during the Quokka's events. Some of the quests appear every time the event is launched, whether you've completed them before or not. Such tasks deal with visiting friends, collecting the harvest, etc. Other quests are available only once, and never appear again after completion. These tasks are mostly connected to the construction of different Quokka-related decorations.
Both kinds of quests go to the Quokka quest counter, however the one-time quests come into account only once, during the time of the current event. Once the event is over, the Quokka quest counter is reset. In other words, if all the one-time quests are completed, but you haven't yet finished the counter quest, you'll have to complete some repeatable quests to finish the counter quest.