Greater surveillance and more boots on the ground are needed to prepare the nation for the threat of a deadly bird flu strain, an environmental group and researcher warn.

Signs of H5 avian influenza have been observed for the first time on Australia's Heard Island in the sub-Antarctic.

Australia is the only continent free from the strain, which has killed millions of birds globally as well as marine life including seals.

Scientists have observed unusual levels of mortality in elephant seals on Heard Island in recent days, with testing needed to confirm whether the strain is responsible.

World Health Organisation avian flu researcher Michelle Wille believes it is inevitable H5 will reach Australia's mainland given its global spread.

The federal government has pledged more than $100 million to enhance bird flu preparedness and response, noting the strain carries "significant risks for Australia's agricultural industries, unique wildlife and the national economy".