Hey Chris,
The tax situation is not very straightforward. According to the tax office you still have to pay 29% taxes on your winnings (paid per month), no matter where you got your winnings.
There were some court rulings recently though, where the court ruled the players could not be taxed twice, for winning within other countries of the EEA. Also the case brought before the highest court had a similar ruling. Officially you still have to pay though, but you send a letter of complaint with your payment. If you do this, then you can be refunded, once the tax office changes their stance on the matter.
In practice your average player doesnt pay taxes at the moment.
In 2015 there is a planned change in regulation, where there will be a (proposed) 22% tax for license holders. Winnings on licensed rooms would be tax free for the players, just as they are now for winnings you make in the state casino Holland Casino.
Not sure what your current situation is (or if you speak dutch), but it might be worth while to "un-dust" this thread in the dutch forums about the matter: https://forums.pokerstrategy.com/nl/forum/thread.php?threadid=45574 (shouldnt be a problem if you post in EN)
There you also find an example of a "complaint letter" which you could include with your payment if you're paying taxes.
Cheers,
Rene