CANBERRA, Australia — Australia’s worst drought in a century has affected more than half of Australia’s farm and ranch land, making more than 70,000 farmers eligible for special federal relief, the government said Tuesday.

The cost of additional drought aid in interest rate subsidies and farmer welfare payments under so-called exceptional circumstances could be $424 million, said acting Prime Minister Mark Vaile.

That’s in addition to $263 million announced last week in extra government handouts for farmers hit hardest by the drought.

‘‘We are in uncharted waters, if you like, as far as this drought is concerned,’’ said Vaile, who is standing in for Prime Minster John Howard while he is overseas.

Agriculture Minister Peter McGauran said the number of farmers eligible for the emergency support had increased by 10,000 to 72,000.

Much of Australia’s farming and ranching belt in the south and southeast has been in the grip of a severe drought for up to five years, with rainfall levels far below average, causing problems especially for grain crops such as wheat and barley.