Southern motorists are being stung by climbing petrol prices.
The 13th straight rise has sent the cost of fuel to its highest level in six months.
The Automobile Association monitored average national petrol price for 91 octane is $2.37 a litre, $2.46 for 95 and $1.74 for diesel.
Dunedin matches those national average prices, but motorists in Queenstown are paying an average $2.43 a litre for 91 and $1.81 for diesel.
In Invercargill, 91 averages $2.22 a litre and diesel $1.54.
Prices in Invercargill are the lowest in the South Island, something AA principal adviser Mark Stockdale believed was due to price competition sparked by the arrival in the city of discount retailers such as NPD.
''It is probably quite surprising to many people that Invercargill has the lowest prices in the South Island and they wonder why that is,'' Mr Stockdale said.
''They have had NPD move in and open up there. They are a low-cost brand, and they have brought lower costs to that region.''
Dunedin has some discount retailers. Allied sells 91 at $2.33 a litre and McKeown at $2.31.
''There just seems to be more price competition with those low-cost brands in Invercargill than there is in Dunedin or elsewhere in the South Island.''
Mr Stockdale said Queenstown's higher fuel prices were usually explained as being due to its distance from a port.
''You might reasonably expect to pay a few cents more for the freight cost ... but whether the 6c a litre higher cost than Dunedin is all because of that we don't know.
''But in Wanaka at the moment prices are much cheaper than Queenstown.
''You would normally think they would be a similar price because they are a similar distance from a port, and yet prices there are much cheaper.
''Again, we are attributing that to Wanaka having a couple of those lower cost, unmanned station brands, whereas Queenstown has none.''
In Wanaka, both Allied and McKeown sites were charging $2.31 for 91 octane.
''The Caltex and BP in Wanaka are charging $2.519 for 91, so it is a classic case of shopping around.''
While petrol prices have been matching international trends and slowly rising, Mr Stockdale said the increase in discount retailers in the South Island could result in prices becoming comparable with those in the north.
The average Auckland price for a litre of 91 is $2.11, while in Wellington it is $2.23.
Gull has recently announced it will be open a branch in Maheno, and northern retailer Waitomo plans to expand in the South Island.
''More price competition is coming to the area, and that can only be good news.''