Four out of five Australians say they are "comfortable" with government agencies monitoring social media sites such as Twitter and Facebook to detect possible terrorist attacks, a survey has found.

The poll found that Australians are far more dubious, however, about social media monitoring by the private sector for the purpose of targeting advertising or offers: just 27 per cent said they felt comfortable with this.

The survey, carried out by Newspoll for the US-based technology giant Unisys, comes amid heightened concern about the use of social media by Islamic extremists to recruit followers and encourage and co-ordinate attacks.

A recent study by the Australian Strategic Policy Institute found that four out of five known Australian extremists had a social media presence.