In a race second placegetter Cameron Brown described as the “hardest'' of his life, Alexander raced off with his first ever ironman victory on home soil.
The win delighted the large crowd gathered at St Kilda willing Alexander to a home victory with the Sydney father crossing the finish line in 7hrs 57min 44sec.
New Zealander Cameron Brown, who ran stride for stride with Alexander for most of the final marathon run, came home in second place (8:00:12) with Belgian triathlete Frederick Van Lierde third in 8:01:26.
"Congratulations to Cameron Brown," Alexander said. "What a warrior. He pushed me right into the well today."Alexander rated the win in Melbourne as one of his most painful after a poor swim put him on the backfoot from the start of the 3.8km swim, 180km cycle and 42.2km run race.
Australian Clayton Fettell lead the elite men's field out of the swim, covering the 3.8km course in 45min 59 sec.
Fellow Australian Luke McKenzie was second (47:11) with Estonian Marko Albert third (47:13), ironman debutant Greg Bennett (47:18) fourth and local Luke Bell (47:22) fifth.
Fettell, contesting only his second ever ironman, extended his lead in the early stage of the 180km cycle and was around 3min 40sec ahead at the 63km mark of the race.
At the mid-way point of the bike leg, Fettell held a four minute lead on Joe Gambles with the chasing pack, containing all the pre-race contenders, less than five minutes off the pace.
Fettell's time at the front came to an end around 153km into the bike leg as the chasing pack caught the man who had done just one ironman prior to lining up in the Ironman Asia-Pacific Championship Melbourne.
Alexander, who has not raced since winning his third crown in Kailua-Kona, Hawaii last October, was perfectly positioned to take the race when he entered the bike to run transition as a frontrunner.
Alexander entered the transition in eighth place, just 33 seconds behind Victorian Luke Bell who was the first man to hit the run course.
At the 10m mark, Alexander, Brown from New Zealand, Llanos and van Lierde were ahead of the field as thousands of spectators started to gather at the finish line at the St Kilda Sea Baths.
A few kilometres down the road the ironman had become a two-horse race between Alexander and Brown who matched each other stride for stride on the marathon.
But around 7km from the finish Crowe broke away from Brown to set up his victory. "I wanted to be the first man to have my name on the Greg Welch trophy,'' he said.
Brown may have come up short but gave his all in the battle. “It was quite tough. My legs are going to be pretty dead after this. I’ve given it my all. It was a fantastic race,” said Brown.WOMEN'S RACE:
Switzerland’s Caroline Steffan took out what she described as a “home victory” for her, as she is based on the Sunshine Coast.
“This feels like a home victory for me and I hope you enjoy it with me,” she said.
In claiming the inaugural title she also posted the second fastest ever ironman time (8.34.51) to beat the UK’s Rachel Joyce and Australian Mirinda Carfrae.Carfrae said it just wasn’t her day as she had simply felt flat.