By Rhiannon Hoyle 

SYDNEY--While Australia's commodities boom was running hot, it wasn't only mining companies that benefited from the billions of dollars in investment pouring into the country. Airlines got a boost, too.

That has changed as slumping commodity prices and reduced investment prompt fewer builders and pit-workers to travel to remote mine sites. The result: several operators of smaller aircraft, such as lightweight jets and turboprops, are going bust, while others are warning of a severe hit to profits.

The latest casualty in Australia's aviation sector is Skytrans Pty. Ltd., a privately owned carrier from Cairns, in Queensland state, which used to fly to more than 20 towns and cities, including Moranbah, a coal-mining town, and Mount Isa, where Glencore PLC produces copper and zinc from one of the world's biggest underground operations.

The airline collapsed this week as seats on its planes became increasingly harder to fill. Not long before that, Brindabella Airlines Pty. Ltd., based in Canberra, and Vincent Aviation Australia, in Darwin, were forced to hand over the controls to insolvency specialists.

"A swing of 10% in passenger numbers will be all it takes to put an airline out of business," said Irwin Tan, general manager of corporate services for Regional Express Holdings Ltd., known as Rex, the country's biggest regional-focused carrier. Rex's profit declined 45% last fiscal year.

At the height of the mining boom in 2012, resources giants such as BHP Billiton Ltd. and Rio Tinto Ltd. were flying in thousands of workers to Outback towns for weeks at a time to dig up everything from coal to iron ore.

Now, demand for labor has slackened as these companies respond to sharp falls in commodity prices-- iron ore plunged nearly 50% last year, for example--by conserving cash instead of building new mines.

Smaller airlines have faced a number of other pressures that have battered their balance sheets, including intense price competition and, until recently, high oil prices affecting the cost of jet fuel--factors that have also hurt larger domestic rivals Qantas Airways Ltd. and Virgin Australia Holdings Ltd.

It has been a stark turnaround since the days when commodity prices were soaring as demand from industrializing Asia for steelmaking ingredients, such as iron ore and coking coal, was expanding rapidly.

Unable to find enough locals in remote Outback towns to build new mines, and urgently needed export infrastructure, mining companies increasingly turned to fly-in-fly-out--or FIFO--workers from all over Australia and as far away as Southeast Asia, who typically were housed in makeshift accommodation on-site.

Skytrans had been restructuring to cope with the cooling resources boom, which had already reduced passenger numbers on some routes. The slide had made the carrier more reliant on a contract with the Queensland state government's transport department--but the company lost it to Rex after it expired at the end of December.

"Unfortunately, with the contraction in multiple sectors of the Australian economy and specifically in the resource sector, the loss of [that] contract turned a snowball into a wrecking ball," Skytrans managing director, Simon Wild, said as the company announced that all of its 188 staff had been laid off. A sharply lower Australian dollar proved a further headache for Skytrans, whose operating costs were mostly counted in U.S. dollars.

Brindabella Airlines, which had been flying since 1994 and operated about 12 aircraft, ran into similar difficulties before falling into receivership about a year ago. Last May, the Australian division of New Zealand's Vincent Aviation Ltd. also collapsed. When the parent company folded later that year, it cited the impact from the failure of its Australian business, which once flew mine workers across the vast northern Outback.

Rex said mine shutdowns and layoffs as commodity prices tumbled had effectively put the brakes on a once-thriving sector. "It's had a devastating impact on the operators heavily exposed to fly-in-fly-out workers," said Mr. Tan.

Brisbane-based Alliance Aviation Services Ltd., which ferries mine workers to places including Olympic Dam, in South Australia state, where BHP runs a copper-and-uranium mine, is also struggling. Last month, it lowered its annual profit guidance by 38% and said it would reduce its fleet of planes. Matthew Dyer, its chief financial officer, said Alliance would hold off on paying a half-year dividend, and that it also planned to write down the value of its aircraft.

Australia's national flag carrier, Qantas, which also operates some regional services, says it is insulated from the trend somewhat by the scale and diversification of its business. Still, the outlook could get worse when resources investment drops further as multibillion-dollar liquefied natural gas, or LNG, terminals that have been in the making for years are finally completed. Building mines and export terminals requires substantially more workers than the number needed to operate them.

"Regional airlines have really struggled with the subdued economy, and the outlook isn't much better," said Ross MacMillan, a Sydney-based analyst at Morningstar Inc. However, he added that sinking oil prices, which tumbled by almost 50% last year, should help ease matters this year for Australian airlines by lowering jet-fuel costs.

Write to Rhiannon Hoyle at rhiannon.hoyle@wsj.com

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