The packagings still have to meet all rules and regulations. A lot of people think that e.g. a UN-approved[1] packaging for dangerous goods is automatically approved concerning the EU Packaging Directive. This is not the case. Whoever fills the dangerous goods into the packaging is still obliged to render that he is not able to use less packaging if he has to meet the UN requirements. Since there are not a lot of approved packagings on the market, and since these goods are not transported in large quantities, you have to document that the given UN-approved packaging is the smallest available in the market.
[1] When transporting dangerous goods internationally, it is required that they are contained in a packaging which is approved and meets certain functional requirements stipulated by the UN. This means that the packaging has to be UN-labelled.