Not quite sure what you're saying here - if you pay not much more than apprentice wages you can get an extra 13 weeks work, but no-one is prepared to take the job?
This is the main rort with 457 visas (and H1B in the USA) - advertise the job at a pay rate where you don't get any applicants, then import someone who will work for the lower rate.
Related is the problem where no business is prepared to train someone. Apprentices are not supposed to be cheap labour, they are an investment in the future of your company, or at least your industry.
By far the highest users of 457 visas is the IT industry, where this is very common. An unwillingness to train, and an unwillingness to pay reasonable rates mean there are a lot of people looking for work while people are brought in on 457 visas.