Posting is made possible for temporary tasks to be executed in another Member State, as such the posting is by nature temporary.
After 12 months (or 18 months with a reasoned notification)[1] your position as far as employment and working conditions is concerned is equated to Dutch employees (see applicable work conditions)[2].
[1] hyperlink reasoned notification to ‘duration of posting’
[2] Hyperlink applicable work conditions to ‘applicable work conditions’