"I wonder if there is any direct connection between the recent lack of wage growth and increased 457s?"
"There absolutely is, and it's not just 457s but all types of skilled migration including skilled migrants moving to Australia permanently."
I doubt there is any connection. When the largest employer sector (small business) has generally been doing it very tough for a good number of years due to things including high Ozzie $, weather events and flow on effects of the GFC. There has been genuinely a lack of money for pay increases for many. More recently the slump in the mining sector took of demand for all types of workers, so competing sectors have not had to increase salaries to retain staff. Then the massive employer our government/s Public Service sector, have not in most cases had pay increases for 3 or more years (why the recent strikes by boarder staff). If the government are not increasing salaries at all then it is not likely the private sector will need to either.
Both the 457 and other skilled migration visas are for listed occupations only - there is no indication that occupations that are not listed or never have been listed, have any higher wage growth rate than any listed one.
Real sustainable wage growth required certain conditions, some of those conditions are controllable by the employer, some controllable by our government/s and some are dictated by international conditions.
Personally I also think the big 2 and the big 4 have also had an impact on wages growth (lack of) as well as the tax practices of the google, apple and Microsoft (and others) use to get money offshore.