What is a 457 visa?

* The 457 visa program allows business to employ foreign workers for a period up to four years in skilled jobs where there is a shortage of Australian workers.

* The numbers are uncapped and there is no limit on the number of times they can travel in and out of Australia.

* Foreign workers can bring their families to work or study.

* Employers must apply to sponsor a 457 worker or enter into a formal labour agreement with the government.

* They must show they are training locals.

* As at September 30 there were 95,757 workers in Australia on primary 457 visas and 76,430 secondary visa holders (members of their family).

* But numbers are in decline - in March there was a 9.2 per cent drop in primary 457 visa holders compared to 2015.

* In July 2013 new laws came into effect, including giving Fair Work Australia the power to investigate breaches of the system.

* Labor wants jobs to be advertised locally for at least four weeks before a foreigner is sourced.