Inalto Ceramic Cooktop Wiring
There's just not enough power there

A 15A 240VAC circuit can only supply 3.6kW, or slightly under half of what your cooktop draws.  You'll need to have a bigger radial circuit put in, dedicated to the cooktop/cooker setup; I presume a 32A circuit would be a standard size in Australia, provided you are served with a single phase.