aftertax superannuation contributions taxed

I was wondering if you contribute superannuation after tax do you also pay a tax rate of 15% to invest it into super?

So effectively some people close to retirement would be better off not over contributing with after-tax income?

Like the only benefit of paying the extra tax and over contributing super is to not pay capital gains tax on it. So you need a few years to beat the 15%.

Like if you retire next year it is pointless to put in after tax income into super because the yearly return will probably not beat the 15% you pay to get into the fund.