"It was quite strange hearing tsunami sirens going off all night," he told the Courier.
"It was certainly not the ideal preparation for a world championship event."
An earthquake off British Columbia sparked the alarm.
The rougher than usual ocean saw competitors encountering challenging two metre waves during the 1500m swim leg.
"I came out of the water still feeling as though I was bobbing up and down," says Shelgren
The New Zealand representative was nonetheless 'very satisfied' with his overall time of three hours 47 minutes, having improved on the bike and run this year despite the course being longer and tougher.
It was his third appearance at the annual Xterra worlds in Maui.
His split times were 32 minutes flat (swim), 2.08.00 (mountain bike) and 59.00 (run).
All his training up the the hills on Sainsbury Road, Pirongia and Loop Road, Te Pahu had stood him in good stead for the mountain bike leg which had 1000 metres of climbing in the first 10km.
"It was a much tougher bike course - if the organisers could find a hill, they put it in this year's course," quipped Shelgren.
There was no let up on the run leg. From transition, it was straight into 300m of climbing all in the first kilometre.
In his two previous appearances at the Xterra, Shelgren finished mid-pack in 2010 and seventh out of 27 in 2011.
He was hardly prepared for the 2010 worlds, having suffered viral vestibulitis (long term vertigo) during winter training. He spent a night in hospital to rule out a brain tumour from his previous cancer history, then spent the next four months barely able to do anything, feeling dizzy all the time.
This season he was fitter from also training for the ITU sprint race. Plus he got a new mountain bike in 2012 (29 inch wheels) which has helped to improve his mountain biking.
Next year he will be racing in the 60-64 age group. He would have been fifth this year in Auckland and third in Maui had he been six months older.
He cannot wait to have another crack at qualifying for the world champs.