Your sparkie is either telling fibs, or you are using something other than the generic HPM or Clipsal switch plates.
We did a kitchen reno about 12 months back including new appliances, and had to install the isolation switch for the electric oven. It's a 30amp switch as well. I went to John R Turks and got the 30amp switch and a double GPO with an extra switch in the middle. Pop the 10amp switch out, push the 30amp switch in, perfect fit. Double-checked with sparkie and a Council building inspector - both gave it the thumbs up.