Multinational tax measure uncosted: govt

The Abbott government didn't include revenue estimates for its measure to tackle multinational tax evasion in the budget because it didn't cost them.

Announcing measures to tighten the tax net on 30 multinational companies last month, Treasurer Joe Hockey declined to say how much this would raise because he didn't want to make the same mistake as Labor and bank money that is "not identifiable".

Under questioning by Labor senator Sam Dastyari, Finance Minister Mathias Cormann confirmed in a Senate estimates hearing on Tuesday the government didn't have the measure costed because it didn't have the necessary information to "credibly do that".

Later, shadow assistant treasurer Andrew Leigh said he was astonished that the government would propose a significant change to tax law without having any idea of its implications.

"This may be the clearest example yet of Joe Hockey's erratic and slapdash approach to managing Australia's economy," Dr Leigh told AAP.