The airline industry pretty much lives on the concept of "Publish Fares". These are fares that are available through all sales channels - through the airlines website, through travel agents, through third-party websites, and everywhere else. This means that the same fare is available to business travelers (who are likely willing/able to pay more for a particular flight, and/or on a particular airline) and for leisure travelers who will only fly if the price is right.
As a result, some companies started doing deals with airlines to sell some flights cheaper - but targeting only specific markets. Basically they wanted to be able to sell to the people who would only be flying if the price was right, without having to reduce the price for the business travelers. One of the mechanisms they used for this was "hidden" or "secret" deals, where you don't get told who the flight is with, or the exact times of the flight (and often even where it's via) until after you've actually booked and paid for the ticket.
Business travelers will generally avoid these options due to the lack of flexibility - they normally want to travel with a specific airline, and they normally want to know the exact times they will be traveling. But for many leisure travelers, the unknown factor is worth the additional savings! These flights are normally not eligible for any "extras", such as frequent flyer miles, upgrades, standby, etc - although that can vary from airline to airline.
Within the industry these fares are normally known as "opaque" fares as you don't find out the details until after booking. They are very common for hotels (Priceline and Hotwire being the two best known, but there are others such as Wotif which I believe does these in your homeland), as well as for car rental. They are less common for airlines, but as you've discovered they do exist!