Outlined in this paper "Risk Assessment of Fluoride Intake from Tea in the Republic of Ireland and its Implications for Public Health and Water Fluoridation"
It's both the water and the fluoride content of the tea leaves, although the tea leaves are the main contributor. The first table in the paper shows this.
Edit: spelling and comprehension corrected; pre-tea morning fog.
> The Republic of Ireland (RoI) is the only European Country with a mandatory national legislation requiring artificial fluoridation of drinking water
So, only a problem in countries that do fluoridation. (I didn't know that that was the case in Ireland. This definitely reduces the chance of me ever moving there.)
The study found that the fluoride already present in the tea leaves was the main source of exposure. Those tea leaves were mostly imported from East Africa, as is the case for many cheap tea bags. Therefore drinking a lot of cheap tea bag tea in New Zealand is potentially not much better.
Outlined in this paper "Risk Assessment of Fluoride Intake from Tea in the Republic of Ireland and its Implications for Public Health and Water Fluoridation"
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4808922