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A vigilant cyclist on Auckland's Northwestern Cycleway was able to avert danger after vandals created a lethal roadblock using rocks.

The popular bike route was closed off by a row of small rocks behind a blind corner and on a downhill section near the Patiki Rd underpass, between Te Atatu and Waterview.

The cyclist who spotted and removed the blockage told other cyclists to keep an eye out on the cycleway for more barriers.

Bike Auckland was notified of the incident this morning by the cyclist, who cleared the rocks immediately, this morning.

"Luckily I was going [the] other way so had time to stop. If I/someone was coming downhill it could have been very nasty.

"There are some serious dick-heads out there," they said.

Ugh. Who does this? This has been cleared away, but please look out if you're traveling the NW cycleway. (Attention @AklTransport @nzpolice) pic.twitter.com/QR7xfOHOjz

— Bike Auckland (@BikeAKL) August 21, 2018

Bike Auckland spokeswoman Jolisa Gracewood said as far as they know, nothing like this had happened before.

"Whether mischievous or malevolent, it's a silly and dangerous thing to do, and someone could have been seriously hurt ... we firmly hope it's a one-off.

"Hundreds of people enjoy riding the Northwestern Cycleway across the causeway every day without incident and value this safe route enormously.

"It's a jewel in the cycle network since being upgraded a couple of years ago, and especially at rush hour is often the fastest way into and out of the city," she said.

Auckland Transport spokesman James Ireland told the Herald they want everyone to be safe when using cycleways.

"Putting rocks across a cycleway is incredibly dangerous and irresponsible.

"We are glad no one was hurt. And if anyone sees anything like this in the future please get in contact with AT straight away," he said.