Alonso’s McLaren was sent twirling through the air when he hit the Haas of Esteban Gutierrez on lap 17 and lost all four tyres and much of its chassis before eventually coming to a halt, upside down.
The Spaniard, 34, was able to clamber free and, while initially disorientated and shaken by the accident, both he and Mexican Gutierrez were quickly cleared of any notable injuries by doctors.
“I am lucky and thankful to be here. It was a scary moment and a scary crash,” said Alonso, who won back-to-back drivers’ titles with Renault in 2005 and 2006.
“You are not exactly aware where you are. You are just flying and then you see the sky, the ground, the sky, the ground. Then I didn’t know where I was because I was so far from the track. Everything happened slower than on the outside. You want to stop and it doesn’t stop, it keeps going and going and going. It was quite scary.”
Defending champion Hamilton, meanwhile, classed his second-placed finish as a positive despite starting on pole and losing ground to title rival Rosberg in the season’s first race in Melbourne.
“We have 20 races from here and there is a long way to go,” said the Englishman. “I just look at 2014, when I retired: I was 25 points down from this race so this result is a blessing. Seven points behind? I am pretty happy with that. Today I had a race. I was behind people, and I had to race. That is what I live for.”
Hamilton slipped to sixth after a dreadful start and was seventh when Alonso’s crash caused a stoppage but benefited from a brave switch to a one-stop strategy and held off Ferrari’s Sebastian Vettel.
Red Bull’s Daniel Ricciardo was fourth at his home grand prix, ahead of Williams’ Felipe Massa and Romain Grosjean, of new American outfit Haas. Renault’s Jolyon Palmer was 11th on his Formula One debut, while fellow Briton Jenson Button was 14th for McLaren.