The friends were prompted to take part in the event when they saw the advertising sign and followed up by looking up the website encouraging everyone to "have a go" and decided it was something to work towards.
They will compete in the half-course triathlon, a 200 metre ocean swim, 6 kilometre bike ride and 2 kilometre run.
The temporary maintenance closure of the Coastlands Aquatic Centre had been a setback, although Dunn had been able to train after work in the Porirua pool.
Even so, they found the prospect of an ocean swim different from a heated swimming pool "a bit scarier".
Other women attending the seminar would be taking part in variations on the event, including the full triathlon, short course and team events.
Williamson said the Kapiti Women's Triathlon was the longest-running event of its kind anywhere in the country with Sunday's run being the 34th annual race.
She urged anyone wanting to register to do it no later than Saturday afternoon at the former swimming pool in Raumati's Marine Gardens.