The Pak'n Save 3 Bridges Marathon has experienced more growth under the online registration system trialled last year, as over 600 runners and walkers have been confirmed to sign up for the dash back and forth across the Whanganui River this tomorrow.

A different organising group within the Wanganui Harrier Club took over the 15-year-old event in recent years and introduced the online application format ,where competitors must sign up by today's deadline, rather than "on-the-day" walk up registrations.

It saves time in the morning for all club officials to watch the race and support those taking part.

Using the online system, the 2017 number of 600 was up on the 500 who took part in 2016, and spokesman Rob Conder said they had exceeded it again by this afternoon, a few hours before the cut off.

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"We've over the 600 mark, so that's a real positive."

Last year's event was won by Auckland's Jaden Cairncross in 2h 49m 12s, which was a personal best as he set himself the goal of breaking 2h 50m despite the muggy conditions.

The 10.5km course starts alongside Moutoa Gardens and takes in the Dublin St Bridge, then heads down Anzac Parade for a back-and-forth trip across the City Bridge, before continuing down Putiki Dr to the Cobham Bridge and running back along Taupo Quay and Somme Parade to the finish.

Full marathon competitors do four laps, and top finishers aim to complete the course in just under three hours if conditions allow it.

The full marathon walkers led the festivities off at 6am this morning, with the runners departing at 7.30am.

Property Brokers half marathon walkers and runners (two laps) depart between 8-8.30am, followed by the Let's Go 5km corporate relay, which loops between the Dublin and City bridges, starting at 9am.

The Unichem Whanganui 10km (one lap) race is 10am, with The Hits 5km race at 11.30am.

In its second year, the McDonald's 1.2km Kids' Fun-run-athon for 13-year-olds and under will begin at 12.30pm.