The Abbott government has quietly reopened a visa loophole that will allow employers to hire an unlimited number of foreign workers under temporary working visas, in a move that unions say will return widespread rorting of the system.

As part of a Coalition bid to remove ''red tape'' from the 457 skilled migrant visa regulations, employers will not be penalised or scrutinised if they hire more foreign staff than they applied for.

Removing the "red tape": The Abbott government removes a visa loophole which allows employers to hire unlimited numbers of foreign workers without scrutiny.

Before the loophole was closed in 2013 by the Labor government, companies in the mining, construction or IT industries were hiring hundreds more workers than they had applied for. In one example, an employer was granted approval for 100 visas over three years yet in 18 months had brought in 800 workers on 457 visas.

Unions are appalled by the change, which was made on February 14, saying it undermines Australian job security while reopening a loophole that can easily lead to the exploitation of foreign workers.