Caltex reported a 109.4% jump in half yearly profit, hitting $180.3 million, based on its fuel discount scheme with Woolworths and amid warnings of weakening refining margins.

The retail affiliation has been credited with an 800 million litre surge in refining sales volumes, to 6.2 billion litres over first half, to the 80% surge in average petrol sales at each of its co-branded Woolworths sites.

"Fuel sales volumes at the Caltex sites, which have been converted to the jointly branded Caltex-Woolworths network, have increased by an average of 80%, while nearby Caltex-branded sites have on average maintained year-on-year sales volumes," said Caltex.

Since late April, Caltex and Woolworths have set up 285 co-branded sites. This week, Woolworths said petrol sales (including at co-branded and Woolworths Petrol Plus outlets) rose 28.3% to $2.2 billion in the year to June 27 but competition from Coles Myer's 590 co-branded Shell sites pushed earnings before interest and tax down.