Can I use my I IGCSE to apply for my work and holiday Australian visa? Or should I take a TOEFL or IELTS?
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.
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.
This what is accepted (one being something from Cambridge): https://www.border.gov.au/Lega/Lega/Form/Immi-FAQs/aelt
However, the visa applications are never decided based on one criteria. If your country has low chance of getting visa you better have everything right. It is also likely they will require you to do the tests if they dont like your English certificate. However, I would suggest not taking the risk and just do the test.