If you are employed, you are required by law to carry the Dutch basisverzekering (Basic health insurance). The cost of this insurance is higher than a private student insurance. For more information you can visit the website of the Nuffic. To apply you can contact an insurance company, or you can apply for the collective UU insurance with Ohra (tel. 020-4004040).
Due to these higher costs, it may be more cost-effective to work full time during the summer months rather than a few hours per week during the academic year. In that case you can limit the cost of the higher insurance fee to the three summer months (June, July, August), and you can have the cheaper student insurance for the remainder of the academic year.
If you have a job and do not apply for a basisverzekering, please note that you will be fined by CVZ. The fine is 350 euro, and you will also be charged the monthly fee for the basisverzekering from the date you started working. Needless to say this can cost you a lot of money, so please make absolutely sure that you apply for the basisverzekering when you get a job!