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July 19 (Reuters) - Australian fuel retailer Ampol Ltd (ALD) on Wednesday posted a 54% decline in first-half refining margins at its Lytton refinery in Queensland, impacted by a five-week long outage at the plant and weaker pricing of its products.

The outage at Lytton's fluidised catalytic cracking unit – used to convert crude oil into gasoline and other petroleum products – prevented production of gasoline from late March to early May.

Ampol's Lytton Refinery margin for half-year ended June 30 was $10.29 per barrel, compared with $22.35 per barrel a year ago.

The Lytton refinery produced 2.97 billion litres of oil in the first half, almost comparable with 2.98 billion litres a year ago.

Ampol, however, said that the Lytton refinery margin for June improved to $12.69 per barrel, reflecting improved production mix as the refinery returned to normal operations by the end of May as planned.

The company also said unaudited earnings before interest and taxes on a replacement cost basis (RCOP EBIT) for the first half was A$575 million ($391.63 million). It reported RCOP EBIT from continuing operations of A$693.1 million last year.

"Group earnings also benefited from a full six months contribution from unit Z Energy, with the underlying business performing strongly, despite the extreme weather events experienced in the first quarter," Ampol said in a statement.

($1 = 1.4682 Australian dollars)