ASIC chairman Joe Longo says the corporate regulator can only police a “sliver” of services provided by the big four accounting firms because they operate in a grey legal area where they are neither “true partnerships” nor covered by federal corporate laws.

Mr Longo said last week the big four firms – Deloitte, EY, KPMG and PwC – were “very hard to regulate” due to the “extraordinary range” of services they offer and because they are governed by state-based partnership rules not designed for firms of their size.