One of the following is required (from the Work and Holiday visa page of Australian immigration). The criteria in bold is likely where the IGCSE fits in.
Achieved an IELTS average band score of at least 4.5 based on the four test components of speaking, reading, writing and listening in a test conducted no more than two years before lodging the visa application. For visa purposes you only need to sit the General Training Module.
Achieved a TOEFL score of 133 on a computer based test, or 450 on a paper based test or 45-46 on an internet-based test, no more than two years before lodging the visa application.
Passed the Occupational English Test no more than two years before lodging the visa application.
Completed all years of primary education and at least three years of secondary education at an educational institution in which all instruction was in English.
Completed at least five years of secondary education (high school) at an educational institution in which all instruction was in English.
Completed a post-secondary qualification that required at least two years of full-time study or training, in which all tuition was in English.
Successfully completed, in Australia, at least one year of full-time study or equivalent part-time study towards a degree, higher degree, diploma or associate diploma at an institution or institutions where all the instruction was in English.
However, I assume that passing the IGCSE isn't enough - at least 5 years of your secondary / high school education, in all subjects, must have been in English.
There are also additional education requirements that must be met based on your home country. You can find these from the page above by selecting your country.