A referendum to constitutionally recognize Australia's Indigenous people would fail if held now, opinion polls out this week show, adding pressure on the government to improve its messaging before the date of the vote is announced.

Australians will vote in a landmark referendum later this year on whether they support altering the constitution to include a "Voice to Parliament," an Indigenous committee to advise parliament on matters affecting Aboriginal and Torres Strait Island peoples.

Supporters argue that voting yes will help mend fraught ties with the Aboriginal community and "unite the nation," and that formally recognizing the 65,000-year-old culture will bring progress in areas such as Indigenous health, education, employment and housing.