concessional super contribution is capped at 27500 per year
you can carry forwards unused cap from as early as 5 years ago
If you pay extra super, it will use current years cap first, if not enough then start using the earliest unused cap, so you have to pay more than the current year's cap to be able to use earlier unused cap.
regarding division 293 tax, the threshold is 250,000.
if you income (before tax) + concessional super contribution is larger than 250,000, which means your salary plus super is more than the threshold, you need to pay extra 15% tax for the part of your concessional super that is above the threshold.
Can your super contribution be all above the threshold? yes, If you make 250,000 salary + 27,500 super as a package, all of your 27.500 super is charged 30% tax.
What if you make even more money and contribute more super than 27,500? Remember the concessional cap is 27,500. If no previous accumulated cap available, the extra super above 27,500 will be non-concessional, which pays full income tax 47%. In that case, it might be better to just pay the income tax and invest the money else where, so you can have access to the money anytime instead of waiting until retired. If you still want to pay non-concessional super, there is a cap as well 110,000, which is to do with rich people avoiding higher tax on investment return (capital gain).
One more example,
if you salary is 240,000 and super is 24,000 then you total is 264,000, which is 14,000 above the 250,000 threshold. The 14,000 part of your 24,000 super needs to pay 30% tax instead of the 15% tax.
Therefore it is a good idea to sacrifice your salary to super, as long as the total super contribution is within your super's concessional cap (current year + previous years left over).