@bluerose31 To be clear, I'm am not talking about only kiwisaver, you should be investing outside of that, and I am surprised you are confused. I'll try and be clearer.
To work out the minimum you need in retirement is simple. You work our your current expenses, multiply those by 25 and that is a ball park figure you need saved for retirement. You can then use any number of online calculators to work out how much you need to be investing regularly to meet that goal by your target date.
For your mortgage, if you expect to carry that debt post retirement, you need to have substantially more saved to cover that ongoing expense using the above. However, the chances are, if you expect to still have a mortgage at 70, then having enough to save beside the mortgage to have 25 times your expenses is highly unlikely.
To break it down to its simplest form. If your minimum mortgage payment is $500 & you work out your minimum investment requirement to be $200 a week, you work your budget from this $700 absolute must pay. Then, if after everything else is paid, you have an additional $100 left over you can put this additional to your mortgage to pay that off sooner OR put it into your investments so you have a larger nest egg at retirement or can retire early. But you can't take the $200 minimum investment and put that off the mortgage otherwise you simply won't have enough years to catch up due to compounding.
If you can only afford the $500 to your mortgage, though, you are in trouble. At 70, your mortgage is now paid, but what then? How do you fund retirement? You need to keep working because your paid off house doesn't buy you food or pay for power.
Now is the time to know the answers while you have time to adjust your plan. The later you leave it, the harder it is.
PS those articles are talking about people working in retirement due to not being prepared and the effect of not having enough invested over time for your investments to compound into a meaningful nest egg which is exactly my point.