"I actually thought it was someone stealing our bins, because we've had a bin stolen before," Ms Willis said.

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But it turned out it wasn't the bins they were after, just the rubbish.

And not just the bottles and cans, they wanted all of it.

"I looked out the window and there was a little truck with a couple of men," she said.

"They had all our bins - some were tipped over, some were upside down - and one of them was being tipped into a big white bag."

Then things got really strange when the garbage trucks came to collect their bins, picking them up even though they had been emptied a few hours earlier.

What had happened was a little-known practice of Wollongong City Council - a waste audit.

"The assessment sees 220 households selected at random across our city to have the contents of their bins bagged and brought to our facility for assessment, rather than being collected by a Remondis truck on their bin day," a council spokeswoman said.

As part of the audit residents are not notified in advance, "to make sure that what is put in the red, yellow and green-lidded bin accurately reflects their day-to-day behaviour".

The information is used to improve waste services across the city and identify opportunities for education programs.

This isn't the only way council tracks what we're putting in our bins - the garbage trucks do it as well.

"Council's waste collection trucks do feature some technology that allows for drivers to monitor the contents of each bin as they're tipped into the trucks," the spokeswoman said.

"This technology has a safety focus as it allows drivers to identify when inappropriate items like gas cylinders, which can explode, are placed in household bins.

"Where there are household bins that contain items that pose a safety risk - or where items are repeatedly being put in the wrong bin - the technology allows council to proactively work with the resident."

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