Wherever you go in Australia and New Zealand, you see people who look like they’re auditioning for the next “Survivor’’ series.

The countries are teeming with adventurers of all ages, sizes and nationalities who come to hike, bike, scuba dive, skydive, bungee jump, paraglide, parasail, zip line or otherwise test their fitness, nerve and dexterity.

During a three-week visit Down Under, I went bridge climbing, scuba diving, jet boating, glacier hiking, black water rafting and rolling down a hill in a giant, water-lined plastic ball. My wife joined me in all those escapades except scuba diving and ball rolling, which was understandable since she’s not fond of breathing underwater and gets motion sickness riding in a car.

During our first stop in Australia we climbed to the top of the Sydney Harbour Bridge, one of the city’s most popular tourist attractions. It takes about three hours to reach the apex of the 440-foot steel arch structure via a series of catwalks and ladders. You wear a harness and are tethered to a rail the entire time, so it’s actually safer than crossing a street in downtown Boston.

For our next adventure we went down instead of up, diving at the Great Barrier Reef, a 1,400-mile stretch off the northeast coast of Australia that includes 2,900 reefs, 600 types of coral and more than 1,600 fish species.

A large chunk of the reef has died in recent years due to global warming, but the area we visited was still thriving with brightly colored coral and fish. We spotted orange clownfish (nicknamed “Nemos’’), blue Maori wrasse (a thick-lipped fish with a hump on its head), a gray reef shark and yellow parrotfish, whose granular poop makes up a significant part of many white-sand beaches.

We tried more water sports in New Zealand, taking a jet boat ride on the Dart River about an hour north of Queenstown. After hiking through an ancient beech forest where scenes for “The Lord of the Rings’’ movie trilogy were filmed, we donned life preservers and sped off in a sleek 14-seat boat.

It was cold and pouring, but the mountain views were breathtaking and the high-speed ride was exhilarating. Racing up to 55 miles per hour, our driver steered us inches from rocks that protruded from the shallow river and did 360-degree donut spins that left us drenched – and questioning his sanity.

After renting a car in Queenstown, Pat and I drove almost six hours over winding mountain roads to Fox Glacier for a “Heli-Hike’’– a helicopter ride to the glacier, followed by a hike on the ice. The next morning we took a short bus ride to the helipad, weirdly located next to a cow pasture, for the five-minute ride to the glacier. There, we put crampons on our boots and set off on a 2½-hour walking tour.

It was so warm that day – the summer was just ending in New Zealand — that I opened my jacket to cool off. Our 12-member group even passed one bearded guide who was wearing shorts. He kicked his legs like a Rockette to show off, making the rest of us feel like wimps.

At one point, we all did a rope climb through a glacial cave. Because of my 6-foot-3 height and inflexible back, I kept bumping my head on the ice ceiling, leaving me with a few scrapes and an embarrassed expression. Since trekking on a glacier can be treacherous, our guide constantly used her axe to clear the way for us. All the safety precautions make accidents rare, though our guide did mention a woman who broke her ankle and had to be airlifted for medical treatment.

Near Rotorua, a northern New Zealand city known for its geysers, mineral baths and accompanying rotten egg odor, I went OGO riding, a local invention where you roll down a hill inside a giant plastic ball. You can do it with or without water in the ball. I chose the wet slide down two grass tracks – the 820-foot straight course and the winding 1,150-foot path. (I didn’t have time to test the new Mega course, billed as “the fastest, longest and steepest downhill ball rolling track in the world.’’)

You can lie down inside the ball or try to stand up, which is akin to walking on a banana peel. Every time I tried to stand I flopped on my back, looking like a comedian doing a pratfall.

About two hours west of Rotorua, in the village of Waitomo, we went black water rafting, which is a misnomer because the water isn’t black and you use an inner tube, not a raft. (It’s described as black water because it takes place in a dark cave, and it’s referred to as rafting because frankly that sounds better than tubing.)

After getting outfitted with wetsuits, harnesses, miners’ helmets with headlamps, neoprene socks and rubber boots, we rappelled (our guide used the German term abseil) down 110 feet into a cave dotted with glowworms that made the walls and ceilings look like constellations in the night sky.

We then zip-lined through a narrow passageway and jumped off a platform while holding our inner tubes under our butts, plunging eight feet into the icy water. Once we entered the shallow river, we alternated walking on the rocky bottom in waist-high water, floating on our tubes and crawling through a tight space our guides called “the rebirth canal.’’ In one section the bottom was so bumpy that one of the guides dubbed our walk “the drunken stumble.’’

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It was extremely slippery and I fell several times. To keep my balance, I often had to grab the jagged walls of the cave, leaving me with minor cuts and bruises. You’re always wet, of course, especially after diving headfirst down a short metal slide around the halfway point. The guides advised us to shadow box as a way to warm the chilly water that crept into our wetsuits.

We often saw glowworms lighting up the cave with their greenish-blue tint. They’re still in the larval stage, when they glow to attract food and burn off waste. Sadly, adult glowworms live only a few days because they don’t have mouths and can’t eat – a true killer diet.

The hardest part was getting out of the cave. We had two choices: leave via a gentle path or wind through a labyrinth of slender openings and then climb up a steep, sharp-edged rock wall. I unwisely chose the latter. When I finally emerged, I felt like I had just gone 15 rounds with Mike Tyson.