You do not need a work permit to participate in voluntary work. There are strict rules for what is considered voluntary work.
For work to qualify as voluntary
It must be work that you yourself have chosen to do.
It must be work that you are not paid for.
It must be work that is organised by a humanitarian, non-profit or religious organisation, or by a volunteer centre (frivillighetssentral), a sports club or similar organisations.
The tasks you carry out must be tasks that no one is normally paid to do.
You cannot do voluntary work for more than 30 hours per week.