What is an Expert Witness?

Wikipedia's definition is  "an expert witness as a professional witness or judicial expert is a witness, who by virtue of education, training, skill, or experience, is believed to have expertise and specialised knowledge in a particular subject beyond that of the average person, sufficient that others may officially and legally rely upon the witness's specialized (scientific, technical or other) opinion about an evidence or fact issue within the scope of his expertise, referred to as the expert opinion, as an assistance to the fact-finder.[1]"   http://en.wikipedia.org/wiki/Expert_witness

What is the Code of Conduct?

Expert witnesses should follow an expert code of conduct. The Expert Witness Code of Conduct is based on Schedule 7 of the Uniform Civil Procedures Rule 2005. An expert opinion should be soundly based, complete and reliable. 

Can I write my own report and give Expert Evidence?

This is not recommended. Even if the applicant or respondent is suitably qualified their evidence may not be seen as independent. Seek independent legal advice in relation to this issue.