Yes, there are some high profile and truly horrible cases that get huge coverage here as well as in Australia, understandably. But the homicide rate in Australia is about the same as Canada and the UK, and much lower than eg the US (UN figures). The thing that you didn't mention in your figures isn't the 24 million people, it's the 650,000 backpackers per year, staying an average of 84 days each, and who will often stay in more remote places where risks can be higher as it is easier for people to stay off the grid. I don't think it's ok, at all. But it's worth bearing those numbers in mind at the same time as highlighting the risks of more isolated places to those people.
And I agree that there are no doubt exploitative bosses but the labour would be no cheaper than what is locally available. The reason these visas exist is that they can't get enough locals to do the work, usually. They pay at least minimum wage and employer superannuation contributions on top are compulsory. The minimum wage for over-21s in Australia is AUD17.70 (so around £10/hour). There are no doubt some shit employers out there - as there are here. But it's not slavery, or 'very cheap labour'.
Having travelled a lot, I think there can be a real risk that people let their guard down in countries that feel familiar, but aren't. People who wouldn't dream of spending large amounts of time alone in isolated parts of the UK do it in Australia. The advice from the cop in the article you linked to rings very true to me.
My sole point in my previous post was that the article has enough elements that don't ring true or sound hyperbolic that I'm simply saying it's worth taking with a pinch of salt, and not treating the whole thing as gospel. Do I think that reforms to the visa system would help stop exploitation? Yes, as I said before (and as previous reforms outlawing the 'volunteering' option have done to make it safer). Do I think the work needed to get the second working holiday visa is incredibly hard, in a way that many British backpackers might struggle to understand until they're in the thick of it? Hell yes, particularly given that British farmers struggle to get British people to do agricultural jobs here. I'm not a huge fan of the way the visa is structured in general, because there is definitely a two-tier system to it. But no one is forced to do a second year of travelling round Australia, so I find the comparison with eg the Morcambe Bay cockle-pickers and those exploited by unlicensed gang masters quite distasteful.