i only have australian passport. i am in spain and want to stay longer. my girlfriend of 6 years is here aswell and she has work. what is the easiest way for me to stay here and possibly work aswell
Depending on the nature of your relationship, you may qualify as your girlfriend's "family member" for the purpose of EU freedom of movement rights. That means that you also enjoy freedom of movement rights. Significantly,
Family members do not need a work permit to work, even if they are non-EU nationals.
The question is whether yours is a "durable relationship" (see https://eumovement.wordpress.com/2008/04/28/other-family-members-who-are-beneficiaries/ for a discussion). As you have been together for six years, it seems likely that you would be able to prove that this is true.
Other questions you asked imply that you are a dual national, that is, that you also hold a UK passport. If that is the case, then none of this applies to you, since you enjoy freedom of movement in your own right.