The Australian Taxation Office has cut 4400 jobs in the past 19 months with one staff member telling a Senate hearing many of them were "our go-to" people.

On Thursday, the Community and Public Sector Union told the hearing into corporate tax avoidance Australia's tax collection would be hurt by the workforce reduction – a contrary line to the ATO's official view.

Weakened: The ATO has cut 17.5 per cent of its workforce, claims the Community and Public Sector Union. Credit:Jeffery Chan

CPSU member Cathy Pugh, who worked in the international branch in Sydney and specialised in base erosion and profit shifting, said managers had been taking on more complex jobs from officers.