The former head of Australia’s electronic spy agency has issued an extraordinary warning against Turnbull government proposals to use the agency to smash the computer systems of criminals domestically, saying such a move would “send a shiver down most judges’ spines”.

In a rare public intervention, Ian McKenzie, who headed the Australian Signals Directorate for more than six years, has told Fairfax Media that using the high-calibre agency to disrupt in Australia the systems of cyber-enabled criminals such as paedophile rings and organised crime gangs would be a “fundamental change”.

His comments follow remarks by both Home Affairs Minister Peter Dutton and his department head Michael Pezzullo indicating they are interested in using the directorate this way, though they have adamantly ruled out using it to collect intelligence on Australian citizens.

The agency has in the past carried out offensive disruption attacks against overseas criminals’ computers, though only when there is no hope of an arrest.

Mr McKenzie, who retired at the end of 2013, said disrupting computer systems within Australia was illegal - including in support of police forces.