Motorists in Sydney and Melbourne can save up to $25 a tank by filling up ahead of an expected steep increase in prices by Christmas that could potentially be followed by further rises after Iranian-backed Houthi militants stepped up attacks on fuel tankers.

Melbourne’s cheapest fuel on Tuesday was $1.66 a litre, according to PetrolSpy, while its most expensive was $2.16. Sydney had a bottom of $1.65 a litre and a top of $2.20.