One is lack of unionisation among Australian residents who do not know what they are entitled to be paid or, for that matter, do not know what their overall entitlements are in the workforce. These people might be recent migrants who have been forced to accept incorrect wages and are too afraid of losing their jobs to say anything about it to anyone. Such people are exactly the kinds of workers that unions are designed to protect but many will be falling through the cracks. Even approaching a union might get their employer's back up, and they will be asking themselves if it's worth the risk.
The other group of people who are being paid incorrectly by exploitative employers are international students. Even less well-integrated into the community, such people have very little understanding of what their rights are, and no confidence to seek help from the appropriate bodies that exist in the Australian community, a pace where they anyway feel like strangers. Accustomed to exploitative employment ecosystems in their home countries they just put up with similar kinds of exploitation here. This is where the 7-Eleven workers fit in.
On top of these problems - problems that are part and parcel of a migrant society like Australia, where the participation of unions clearly continues to be essential, for the reasons outlined above - the government now wants to attack penalty rates paid to workers who choose to work on Sundays. This is a little obsession among parts of the libertarian right in Australia, and is designed to get us used to accepting a more unfair workplace generally, along the lines that have been pioneered in America, where low-wage workers earn less than half what they do in Australia.