I am unsure about the current design poly grad pay.

But for 28% (Close to 30%) increment for the job for a poly diploma, i feel it's the quite good increase at current market. (I know some Design Degree grad getting less )
Maybe you need to calculate everything include bonus and other benefits to get the good comparison.

Sometimes it's not only about the pay. You may want to double check on job scope and find out more about the working culture, does it suit you?

Just my 2 cents.