The plane took off at 6.21am and headed north-west, towards the clouds. About 200km from Hobart airport, above Tasmania’s central plateau, there they were: clouds ripe for seeding.

Thursday’s flight was urgent. Tasmania has just experienced its driest spring on record, followed by its hottest summer. The lakes that supply the state-owned hydroelectric power scheme are at record lows, just 12.8% full, and the Basslink cable that provides back-up power from the mainland has broken, forcing the state to ship in 200 diesel generators. Tasmania’s energy crisis is a glimpse of an uncertain future for Hydro Tasmania. It seeds the clouds and hopes for rain.

Silver iodide dispersed from the back of the little Cessna aircraft theoretically makes the below-freezing water vapour in clouds above the Tasmanian highlands form into ice crystals, then fall as rain. But it is an inexact science.

This week it worked, a bit. The cloud-seeding run covered the upper Pieman catchment, which feeds into Lake MacKintosh; the Mersey-Forth catchment, flowing into lakes Rowallan and MacKenzie; the upper Derwent, feeding lakes St Clair, King William and Echo; and Great Lake. Rain gauges in that area subsequently recorded up to 9mm of rain, with the exception of Great Lake, where the rainfall was too low to register.

“It certainly would have made a difference but it’s hard to know how much of a difference,” Steve Davy, chief executive of Hydro Tasmania, tells Guardian Australia. “We would never know how much rainfall would not have occurred if we didn’t do cloud seeding.”

Fortunately there is more rain on the way. The Bureau of Meteorology predicted the highlands could get as much as 50mm in the next week, in what looks to be a return to average late autumn-winter rainfalls.

Davy is confident rain will refill the lakes to a manageable level and that the dry spring was an aberration, caused by El Niño weather patterns. With Basslink set to be repaired by June, and the prime minister, Malcolm Turnbull pledging to fund a feasibility study into building a second cable, it may soon seem like business as usual.

Tasmania is drier than when the Hydro-Electric Commission was founded in 1929, but Davy says that since the 1990s, considered the start of the “new normal” of slightly lower Tasmanian rainfalls, Hydro has not observed a drying trend.

“From last spring, summer and early autumn it has been a very dry period, the driest that we have seen,” Davy says. “We don’t know yet if that’s an outlier or the start of a new drier period.”

The super-cool water droplets needed for cloud-seeding form cumulus clouds with a distinctive cauliflower shape, like this one. Photograph: Hydro Tasmania

He suspects it is the former. “If an average year is going to be like the year we have had, that’s quite a downward change,” he says. “There’s not evidence for that, that this is the new normal.”

But a CSIRO research scientist, Dr Michael Grose, says there may be very significant changes in rainfall across south-eastern Australia if worst-case climate change modelling plays out.

In Tasmania, Grose says, that may mean a decrease in spring and summer rainfall of between 15% and 25%, contributing to an annual decrease of 10%.

That could lead to more summer bushfires on the usually damp west coast, like those that devastated the Tasmanian Wilderness world heritage area last summer.

“This present season may be something of a taste of what could be the impact of climate change in the future,” Grose says.

Low spring and summer rainfall cannot be boosted by cloud seeding, which relies on super-cool liquid water droplets that hang in clouds around the Tasmanian highlands and the Great Dividing Range on the mainland, where clouds are seeded by the Snowy-Hydro scheme.

Monash University associate professor Steven Siems says the practice, though largely unsuccessful in other areas, had been shown to add a significant amount of rain in Tasmania: an analysis of rainfall between 1964 and 2005 found that seasons when cloud seeding occurred correlated with a 5% to 10% increase in average rainfall.

Hydro Tasmania already factors that increase into its long-term modelling.

Modelling produced with Hydro Tasmania’s involvement as part of the Climate Futures for Tasmania project found that average inflows into the state’s water storages would decline over the next 100 years, particularly on the central plateau, and that could have a “marked impact” on power generation.

Davy says that gap could be filled by wind and solar, with the hydro scheme providing a firm baseload.

But at the moment Tasmania remains in an energy crisis. Hydro is talking to ecologists and the recreational fishing industry to determine how low it can draw the lakes down before the ecosystem is damaged. Heavy industry plants, such as the paper mill Norske Skog and Bell Bay Aluminium, have agreed to reduce their production to ease the burden on the electricity network.

Hydro has turned on the gas-fired Tamar Valley power station, which the state government put on the market in August, and has begun shipping in up to 200 diesel generator units, which the state treasurer, Peter Gutwein, says would cost $44m to lease and install and a further $11m a month to run.

On the shore of Great Lake, Kaylee Hattinger, who runs the local pub with her husband, Peter, has spent the past four months counting tree stumps.

“If I haven’t taken any notice for a few days, the next time I look there will be another stump out of the water,” she says. “I don’t know how they are going to fix it … El Niño was forecast for years. How could they not have planned for this?”

It’s an 18-metre walk between the shoreline and the water now. Boat ramps jut into nothingness. In 11 years, Hattinger says, she has never seen it this dry.

The lake is stocked with brown trout but Hattinger says fishermen, unless they are professionals with “big-arse four-wheel drives”, cannot get close enough in the slippery mud to cast off, and that is bad for business. There is still plenty of water in other highland water courses but Great Lake, a major Hydro reserve, is less than 11.4% full.

“The red wine brigade – that’s what we call the fly-fishers – they have got plenty of water in their lakes,” she says. “But the beer drinkers don’t.”

Great Lake was a series of wetlands around a smaller freshwater lake before it was dammed in 1967. With a surface area of 176 sq km it is Australia’s largest permanent natural freshwater lake. Lake St Clair, about 50km west, is Australia’s deepest natural lake, and the combined Lake Pedder-Lake Gordon system, in the world heritage-listed west coast, is the country’s biggest man-made water storage.

These lakes are the main storages for the Hydro Tasmania network, which can generate 9,000 gigawatt hours of renewable energy. Tasmania’s annual requirement is about 10,800GWh.

In an ordinary year, Davy says, another 1,000GWh was provided by two Hydro-owned wind farms, Woolnorth, off Tasmania’s north-west coast, and Musselroe, in the north-east. The balance is purchased from mainland providers through the Basslink cable. Or it would be, if the cable hadn’t broken in December.

Davy says Hydro Tasmania has been prepared for a possible fault in the 290km cable, which is also the internet connection for customers not with Telstra, but its risk management planning assumed any fault would be fixed in two months. It has now been four, and is expected to be six before the fault is repaired.

“Up until this fault, Basslink was the best performing cable in the world,” its chief executive, Malcolm Eccles, told the Launceston Examiner this month.

“We’ve got a cable fault that’s happened after 10 years and the rest of the performance seems to be forgotten.”

Until it’s fixed, Tasmania – like an old car – will be running on diesel.

• This story was amended on 2 May 2016 to clarify that Hydro Tasmania’s 9,000GWh output comes from its whole network, not just the lakes mentioned by name.