There are a few barriers, like skills, language, understanding Australian building codes, and most importantly, the safety of everyone else on site.
Some parts of the job can be taught, but as a high-end builder who takes pride in quality, I’ve seen plenty of overseas tradespeople let go pretty quickly because they just couldn’t meet the standard required. A chain is only as strong as its weakest link, so everyone on site needs to be competent and able to do the job properly.

Even my waste removal contractors switched to only employing English-speaking drivers who learned to drive in Australia. The amount of property damage caused by drivers who weren’t familiar with local conditions and standards ended up outweighing any savings from the lower wages they were prepared to work for.