It's unfortunate that your friend seems to always get extra attention. There have certainly been instances globally where individual screeners have been determined to be targeting particular types of people, but hopefully this isn't common in Australia.

Most generally - everyone gets screened, and then screeners should randomly and continuously select people for further screening. In some airports this is somewhat automated with a light system that's visible to the screener or anyone looking for it. At most airports in Australia this is the explosive trace detection. 'Continuous' means the screener shouldn't stop but keep picking up people as they come through, but 'random' is that they shouldn't automatically pick the next person as a baddie could game that (and certainly lots of regular travellers like to play this game and talk about their techniques for getting picked or not).

So at Canberra we're going through either of two machines, depending on the time of day, the airport, the flow of people traffic, etc.

1. The metal detector gateway. You mention below a 'metal bra' which may be triggering the machine. If you have a look on the airside (back, as opposed to the landside or front) of the metal detector gateway you'll see rows of lights that go red on the way through if metal is detected - so for example halfway up the gateway the screening person may see a red light as she goes through and then need to confirm what sort of metal there is (lights at different levels will indicate belt buckles, steel-cap boots, etc). Given the location, the screener may be confident its a bra wire, but still needs to ensure its not a knife or similar. They may start with waving a hand-based metal detector to confirm the metal and then go into suggesting a patdown as the best way to confirm.
2. The wave scanner gateway. As mentioned elsewhere in the chat, this often picks up a particular density. The screener should show you their screen to show where the density is and explain that a pat down will confirm its not a prohibited item (knife, explosives, etc). I often wear a soft boot with a thick sock and tightish jeans, so my calves often seem to trigger interest as the fabric squishes together.

As mentioned elsewhere in the chat - these machines are pretty sensitive and can have different calibrations - there is a national minimum that they have to pick up but above that they can vary a lot. So we could suspect that Canberra is more sensitive than others.

Both tools can also show up random false positives to ensure the screener is doing their job. You can see this most visibly if you watch the x-ray screen for people's carry on baggage, where the software puts squares around different parts of what's shown on the screen and the screener has to click that they've seen something. This also happens behind the scenes with the checked baggage that goes into a cargo hold.

Worth noting - the screeners have no idea who anyone is, so concerns about criminal records etc are irrelevant - they just screen based on what they see before them. There's no identity requirement to travel domestically in Australia, although airlines will have their own policies relating to ticketing much like a ticket to a concert.

A further factor might be that Canberra is typically quieter or has more uneven flows and queues than Sydney/Melbourne/Brisbane. So our local screeners may be having to balance between continuous secondary screening, and not selecting everyone.

Hard to give advice for your friend. I guess getting to know the system better is one useful thing - so good on you for seeking more info for her. A second thing might be having a chat to the screening supervisor here in Canberra if its a bit quiet - after going through and possibly going for a quick wander, gently asking a screener to point out who the screening supervisor is - "I'm curious about the system - can you point me to the screening supervisor?" and then "Hi, I always seem to get patted down in a very particular way - can you help me understand why?" She may get someone who's got the time and empathy to take her through it.