Anti-discrimination authorities are being urged to investigate a flood of online job ads offering positions exclusively to temporary migrants on work visas.

The Victorian Equal Opportunity Commission is considering a landmark investigation into whether the ads – offering jobs only to overseas workers – are in violation of state anti-discrimination law.

Trade union leaders say the ads are discriminatory because they prevent local workers from applying for jobs on the basis of their race. Under state law, the definition of race includes a person's nationality.

Led by the Construction, Forestry, Mining and Energy Union, the strong calls for an investigation come as new data revealed Victoria's jobs market to be the least promising in the country, with more than six jobseekers per vacancy.

Online advertisements offering jobs specifically to migrants holding or capable of obtaining 457 or other temporary work visas are rife on classifieds websites such as seek.com.au and Gumtree.