There are lots of ideas in your introduction but they are not forming an indication of your main arguments. I'd suggest you develop a stronger opening sentence e.g. "The role of plastic in environmental pollution has forced a change of approach to single use plastic bags." As your topic is "the effectiveness of the plastic bag ban in Australia", you're writing about "why is the ban necessary", instead of "is it effective". So you need three main arguments to support your contention that this ban is effective. Making consistent links with consumer behaviour could give your writing a stronger connection to "is the plastic bag ban effective".You could "flip" the content of your first body paragraph to focus on the positive effect that a change in consumer behaviour can have, then follow this up with the alarming statistics about how long degrading of plastic actually takes. In your second body paragraph you could also make a stronger distinction between plastic in landfill and plastic waste in the waterways and oceans - this is the invisible consequence of the "lazy consumer behaviour" you've mentioned. This is where you might mention the supermarket back down sending contradictory messages to their customers - the support of these big retailers is an important part of the change in consumer behaviour. Your third main argument (about waste of petroleum resources) generalises too much and could instead focus on renewable and reusable resources to replace petroleum based plastics and that these products that replace single use plastic bags are readily available. Australians can follow the models developed by other countries with similar bans.