From the article:

"To understand why, consider someone who spent three months of the year receiving unemployment benefits while they searched for a job, then worked for nine months. The government considered their income for the entire year and averaged it, then assessed their eligibility for payments based on their average fortnightly earnings – not their income when they were out of work and therefore eligible for payments."

Because the Australian government averaged the yearly income to see if the average fortnight was below the mark. Instead of following the law which gave benefits to those who had low income for a brief period.

This was not a software issue. This was a willful top-down policy choice to use bad math. (At least, so the article says. I don't live in Australia.)

What was also amazing about it, is they would use your old address when you received welfare payments, but not update this from other systems such as the tax system. This meant that the debt letters were being sent to the wrong place, and then eventually handed over to debt collectors to chase. Credit scores were impacted because of this.

So actually a matter of lack of unique id (and a properly registered address), then. Huh.

@anigbrowl - go read this comment

No you're not understanding. There is a unique ID, in this particular context it's called a CRN (customer reference number). Every person who receives social security in Australia has one and they're fixed for life.

The lack of unique ID has nothing to do with this. This was government blatantly attacking poor people through willful mismanagement of an automated debt inferment system in conflict with the legal process through which that debt should have been calculated.

They 100% knew what they were doing and did it deliberately.

Well.. math works differently down here, after all..

"The laws of mathematics are very commendable, but the only law that applies in Australia is the law of Australia" (fmr PM)

That's what the article is about, go read it