Gull petrol stations have launched a birthday celebration discount, dropping fuel prices by 21 cents per litre today and tomorrow.

"We're celebrating our big 21st birthday," the company wrote on its website.

Pump prices at all Gull sites are lowered by 21 cents per litre of fuel from 7am this morning to midday Friday, the company says.

Caltex Australia, which no longer has any ties with Texan giant Chevron, bought Gull in 2017 for $340m and this year had its first annual contribution from the Kiwi challenger petrol brand.

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The group generated revenue of A$559.1m ($581.6m) from New Zealand in calendar 2018, up from A$203.5m a year earlier, when Caltex bought Gull. Gull contributed to Caltex's 39 per cent increase in international fuel sales volumes to 3.5 billion litres, alongside the group taking over supply management for Seaoil Philippines and more third-party sales.

International earnings before interest, tax, depreciation and amortisation soared 172 per cent to A$75m, compared to a 5 per cent increase in ebitda for the dominant Australian fuel and infrastructure arm at A$435m.

In August, Caltex said Gull was growing in line with expectations after the first half.

The Gull 87-strong network consists of 63 controlled retail sites, including 40 that are unmanned, and 24 supply sites. When Caltex bought the challenger brand, there were only 77 sites.