The Diploma for Graduates (DipGrad) consists of 100-, 200-, and 300-level papers. You need to be a graduate - or close to completing your degree requirements - in order to do a DipGrad. The DipGrad is an undergraduate level qualification.
A postgraduate diploma, such as the Postgraduate Diploma in Science (PGDipSci), consists of 400-level papers only. Postgraduate diplomas are postgraduate qualifications - you will need to have completed an undergraduate degree to be eligible for admission to these programmes.