Division 293 charges an extra 15% on concessional super contributions for high earners. It applies when your "income for surcharge purposes" plus your concessional contributions exceeds $250,000.
The "lesser of" rule
The extra 15% applies to the lesser of your concessional contributions or the amount you're over $250,000. This is the bit most calculators get wrong.
Example: $260,000 taxable income + $30,000 contributions
Income for surcharge purposes is $290,000, which is $40,000 over the threshold. The Div 293 base is the lesser of $30,000 (contributions) and $40,000 (excess) = $30,000, so the Division 293 tax is $4,500.