How the KiwiSaver check works
Five layers, from the payslip outward. Each either reconciles, does not reconcile, or tells you exactly what figure it still needs. The checker never says anyone underpaid anyone — it says which figures do not line up and gives you the question to ask, in a letter written to keep payroll on side.
The two traps that cause false alarms
ESCT. Employer contributions are taxed before they reach your account, so the landing amount is meant to be smaller than the headline rate — roughly 2.8% net on a stated 3.5% for a middle earner. Compare the gross rate against your account and you would conclude, wrongly, that every employer in the country is short-paying. The band is set by last year's earnings and is fixed for the whole year: a raise in June does not move it.
The transfer lag. Money moves employer → IRD → provider, and that takes weeks. The checker builds the lag in explicitly: a gap smaller than the money still legitimately in transit is normal, and is reported as normal.
What it will not do
It will never ask for a myIR or provider login. Typed figures only. It cannot see your employment agreement (relevant for total-remuneration packages) or inside IRD's processing — where those matter, it says so instead of guessing.