is there any way to get a port of entry stamp in Spain. specifically Mallorca. i need it in my UK passport to apply for a nie (tax number). i entered on a Australian passport
No. Citizens of the European Union do not get passport stamps when they enter other European Union countries.
Because of this, it is extremely unlikely that anyone would tell you that you need such a stamp for your application. Where did you get this information?
As an EU citizen, in fact, you can apply for a residence certificate and NIE at the same time, although the residence certificate is not required. For a discussion, see http://spain.angloinfo.com/moving/residency/nie-the-application/ and http://spain.angloinfo.com/moving/residency/.