Second Bachelors in Maths after Masters in Optics for Theretical Physics Phd
So is it a good idea to do a second bachelors degree in Mathematics(distant education) before applying for a Phd in theoretical Physics in Europe/Australia ?


In general, getting another bachelors degree after getting a masters degree is not seen very favorably.  I understand your though process - show the admissions committee you'll do grade-A work in math.

In reality, it's much more likely to signal to the admissions committee you don't have a clear purpose, and make them wonder why you didn't just switch to a masters in math.  Furthermore, since you already have an advanced degree, they are less likely to be impressed you went back to undergrad and aced another bachelors degree.

The admissions committee is most likely to weight your academic performance doing your masters degree heavily.  If you are truly set on switching fields, I would recommend you get another masters degree in mathematics OR applying for a PhD in mathematics.