May 18 (Reuters) - Australia's top construction materials maker Boral Ltd said on Wednesday that annual profit may come in nearly a third below expectations as "extraordinary" rainfall slowed sales and rising fuel costs trimmed margins.

The Sydney-listed company already warned in March that heavy rain across Australia's east coast, where most of the population lives, would hurt sales volumes, and flagged pre-tax profit of A$145 million to A$155 million for the year to end-June, from A$140 million the previous year.

However, in a share-market filing, it said trading since then had been "impacted by continuing exceptional rainfall and inflationary cost pressures" and it now expected "additional adverse impacts" to annual profit of about A$45 million.

Two-thirds of the adverse impact was caused by heavy rainfall, which has caused heavy flooding and shut down construction in the states of New South Wales and Queensland, the company said.

The other third was caused by inflation, mostly energy costs. The company added that it expected the elevated costs to continue to the end of the financial year.

"We are responding to this challenging operating environment by implementing additional measures to mitigate the impact of transport and fuel inflation alongside the already announced out of cycle price increases, and accelerating our focus on costs," CEO Zlatko Todorcevski said in the statement.

Boral shares were down 1.3% in morning trade, against a 1.2% gain on the broader Australian market.

($1 = 1.4225 Australian dollars) (Reporting by Byron Kaye in Sydney and Indranil Sarkar in Bengaluru; editing by Uttaresh.V)