MELBOURNE, Australia » Rivers surged to record heights and put rural towns and an already waterlogged city on flood watch Friday in a weekslong crisis that is costing Australia at least $3 billion in lost farming and coal exports.

Residents of Brisbane sandbagged homes again as an exceptionally high tide and overnight rain engorged the main river through the Queensland state capital, already devastated by flooding last week. Mayor Campbell Newman urged people in low-lying areas to be prepared. The Brisbane River is expected to remain high until Sunday.

Australian Associated Press reported flooding of some homes, businesses and a race course in inner suburbs where more severe flooding struck last week. Officials could not be immediately contacted Friday for details.

In southeast Victoria state, hundreds of homes faced threats over the weekend as four rivers northwest of the state capital Melbourne were expected to reach their highest peaks in a century, said Lachlan Quick, spokesman for the State Emergency Service.

Eastern Australia has endured weeks of massive flooding that the government says could be the nation’s most expensive natural disaster ever. It shut down much of Queensland’s lucrative coal industry and has caused 30 deaths.

The Queensland floods will reduce coal exports by 16.5 million tons by March, the Australian Bureau of Agricultural and Resource Economics and Sciences reported Friday.

The government’s main forecaster on the farming and mining industries said the lost exports are worth up to $2.5 billion.

Australian farm produce — most of which is exported — including fruit, vegetables, cotton and sorghum will be cut by at least $500 million in the fiscal year ending June 30, ABARE said.

The global credit ratings agency Fitch Ratings estimated Friday that between 30,000 and 40,000 houses were damaged by the floods in Queensland alone.

Australia is the driest continent after Antarctica and was recently in the grip of the worst drought in more than a century. But 2010 was Australia’s third wettest year on record, despite the country’s drought-stricken southwest region enduring its driest ever year.

With the drought retreating, ABARE had predicted in December last year that farm production would rise by $2 billion from $28 billion last fiscal year.

Also in December, ABARE had predicted coal exports would increase to $48.6 billion this year from $36.4 billion last year due to higher prices and greater volumes driven by burgeoning Asian demand.

The government has yet to estimate the cost of the damage, which will reach billions of dollars, but Prime Minister Julia Gillard said she was considering introducing a flood tax to pay for reconstruction.

Some estimates already were at $5 billion before the Brisbane floods last week.

In Victoria, 72 towns have been affected by flooding already and more than 3,500 people have evacuated their homes.