Australia's marginal iron ore mines have sprung back to life, with two resuming exports in the past fortnight after a seven-year high in prices and a little help from big multinationals.

Signs the bumper prices will extend well into 2021 will also provide the ideal start to life for the $US6 billion ($8.1 billion) worth of new Australian mines that Fortescue, BHP and Rio Tinto will bring into the market within the next seven months.