Motorists travelling the long inland Lewis Pass route in the South Island would be wise to check the availability of petrol.

Mystery surrounds the reasons for Mobil's shortages of petrol for its South Island branches - except they are expected to continue.

Mobil has been economical with its public communications, using spin doctors to prepare brief statements blaming problems with shipping and storage, and unprecedented holiday demand.

Motorists travelling long distances would do well to keep topped up, especially on the long inland Lewis Pass route where Mobil Murchison is the only petrol retailer in the town.

The station currently has some stocks of 91 octane but no 95 octane. However the situation in Nelson has improved where Nelson Bays Mobil has received full supplies of both octanes.

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The proprietors of most Mobil service stations generally refuse to answer questions about their petrol supplies, referring queries to Mobil corporate headquarters, which remain unanswered.

Some forecourt attendants have been more helpful.

Phone calls to half a dozen outlets around Christchurch revealed most had run out of 95 octane and were uncertain how long supplies of 91 would last.

The same was true of at least one Greymouth station, the other refused to comment to news media.

A Mobil outlet operator in Invercargill reported ample stocks of 91 octane but no 95 octane.

"There's no 95 octane south of Timaru at Mobil stations. I heard it was a pumping problem at one of their storage tanks."

The question posed by many Stuff readers is why Mobil seemed to be the only affected brand, given that petrol was transported on the same ships as other brands.

Some motorists were frustrated they would be unable to use their supermarket loyalty discount cards which were due to expire.

AA has advised affected motorists whose cars run best on 95 octane to either use alternative brands, buy 98 octane, or to top up frequently with 91 to keep a blend of the petrol in the tank.

A spokesman said using 91 octane for a 95 octane-tuned engine would be unlikely to affect it if it only happened occasionally.

Mobil's statements over the past week did not provide details about which outlets were affected.

"Oil New Zealand is currently managing the supply of unleaded petrol to our customers out of the South Island terminals," Andrew McNaught, country manager, Mobil Oil New Zealand said.

"With regard to Murchison, we expect to see some improvement to the supply of fuel to service stations in the Nelson region over the coming days, following the delivery of fuel to the Nelson terminal on 14 February.

"While we are receiving replenishments of fuel at terminals during this time, it is likely we will have to continue to closely manage supply arrangements in the South Island throughout the next fortnight and beyond.

"There may continue to be periods over the next week or so when some Mobil service stations in the South Island do not have regular or premium unleaded fuel available. We are actively working to put in place alternative supply arrangements to minimise any impacts on service stations and our customers.

"We've had unforeseen delays in vessel replenishments into Lyttleton, Dunedin and Bluff and a reduction in available port tankage in Dunedin. In addition, Mobil has received less gasoline than planned from Refining NZ for the preceding months of November and December." Mobil said.