The Nuffic Glossary is a list with explanations of terms used in Dutch education.
When applied to master's degree programmes, the term initiële is used to mean that it follows on directly from a bachelor's programme and is fully subsidised by the government. A post-initiële master's degree programme is separate and is often pursued after a period of employment and/or by students who have transferred in from other institutions. It is not subsidised.
Because the distinction has more to do with funding than with level, it might not be at all relevant outside the Dutch national context, except as it affects the tuition fee. Moreover, initiële and post-initiële programmes are at a comparable level within one institution, whereas the English terms 'initial' and 'post-initial' imply that they are not.
Consider not even attempting to translate the term. Instead, describe the features of the programme.