Providing a $1 billion loan to underwrite Adani's proposed mega coal mine in Queensland would have "serious negative impacts" for Australia's international reputation and "unpick the progress" of the Paris climate agreement, according to Christiana Figueres, a former United Nations climate chief.

Ms Figueres has written to the Turnbull government's Northern Australia Infrastructure Facility (NAIF), which is considering a concessional loan for a rail link from the mine to the coast.

The former executive secretary of the UN's Framework Convention on Climate Change

sought to highlight that under the NAIF's own enabling legislation, it "must not act in a way that is likely to cause damage to the Commonwealth government's reputation, or that of a relevant State or Territory".

Ms Figueres warned the expected total lifetime carbon emissions from burning coal from the proposed Carmichael in the Galilee basin would be 4.64 billion tonnes of carbon-dioxide, according to details of the letter obtained by Fairfax Media.