When six towers toppled in Geelong's northern suburb of Anakie on Tuesday this week, more than half a million Victorians were cut off from power.

They formed a crucial part of the more than 65,000 kilometres of high-voltage transmission lines slicing their way across Victoria, connecting electricity generators like coal-fired power stations and wind farms to substations dotted across the state.

From the substations, the power is then shot out through a massive web of smaller distribution powerlines — like the ones standing on the side of a suburban street — and fed directly into homes and businesses.

To do their job, these high voltage distribution towers need to be big, stretching anywhere from 60m to 80m into the sky from top to tail.

So how can they topple?

Why do transmission towers fall over?

The collapse of the six towers north of Geelong was a result of a thunderstorm system passing over the region, producing wind gusts of up to 122kmph.

This is what's being initially blamed for the tower failures, just as it was when another 500kV transmission tower fell in nearby Cressy back in 2020.

But the answer to a simple question isn't as simple as "it was too windy".

In a 2020 report into the Cressy tower collapse, Energy Safe Victoria, the state's energy watchdog, found "severe convective downbursts" were specifically to blame for the tower falling. The term is a neat way of describing how a thunderstorm can violently push a huge burst of wind downwards, which then pushes outwards as a gust front when it reaches the ground.

The ESV incident report found the Cressy towers were built in the early 1980s under an old "overhead line design" standard.

"The latest version of the standard requires designs to consider convective downdraft wind gusts, sometimes referred to as high intensity winds, which are generated by severe thunderstorms," the report noted.

"The original towers were designed to older standards by the State Electricity Commission of Victoria (SECV) between 1978 and 1980 which did not consider convective downdraft wind gusts at this time. The SECV constructed the towers between 1980 and 1983."

A 2020 report by AusNet Services — the organisation charged with maintaining the state's transmission system — found the average 500kV transmission structure was built 41.4 years ago.

It also showed that about a third of the state's transmission towers are now approaching their 60th birthdays.

Looking at this week's tower collapse, it is understood the six toppled towers in Anakie were built in the 1980s.

What checks are done to ensure towers can withstand weather events?

With an average span between towers of 300-500m, it's estimated there are at least 130,000 of these distribution towers across Victoria.

As the company in charge of maintaining Victoria's transmission network, AusNet is required by the Electrical Safety Act to inspect lines to ensure they're in safe working condition, check the condition of tower bases and powerlines, and act quickly to fix problems.

Every year qualified linesmen — and drones and helicopters — inspect AusNet's full transmission line network to "identify line and easement defects, check line clearance space, and determine corrective maintenance priorities", according to the company's Bushfire Mitigation Plan.

A series of further activities is also undertaken by the company to ensure its infrastructure is up to scratch, including workers physically climbing towers for "close up visual assessments".

But these tower climbs are dependent on the location of the major towers, including — every three years — checks of assets near major roads, rail crossings, interstate transmission sites and in regions with high corrosive environments. Towers in urban areas are climbed every six years.

The remainder of the network's transmission towers are typically climbed once every nine years.

AusNet also monitors the corrosion of towers' metallic components, giving each tower a grading to guide future maintenance and replacement.

Why don't we put the powerlines underground?

When we discuss powerlines sparking bushfires or widescale blackouts, this is the question that often comes up.

While the simple answer — it costs too much — is often quickly offered, it's not a silly question at all.

New estates across Victoria are being built without overhead powerlines, and the state government has even funded the undergrounding of private overhead electric lines to mitigate bushfire risks.

But when it comes to the hundreds of thousands of kilometres of powerlines across Victoria, and the roughly 65,000km of transmission lines, the costs are astronomical.

In 2020, Sydney power company Ausgrid did the numbers on undergrounding its entire network — which only takes up a portion of NSW — ringing in the final cost at somewhere between $72-130 billion, and taking 40 years to complete.

"It costs about $2.5 million per kilometre to put wires underground, which is 15 times more expensive than the cost of above-ground wiring," the company said.

"It's also much more difficult and time-consuming to repair."