Mr Rees said the orca got as close as five metres away. "He was going up and down and blowing and stuff like that. I stopped looking back but by the time I got out he was where we were (surfing)," he said. "Then we saw three others out there. They were just hooning it, aye."
It was estimated there were up to seven orca which then came into the river mouth before heading back out to Castlecliff Beach, Long Beach and towards Taranaki.
"They were really jumping around in the river," Mr Rees said. "It was pretty cool."
Ingrid Visser from the Orca Research Trust said they may have come up from Wellington. Orca had been spotted in Island Bay on Thursday and were possibly the same ones spotted along the Wanganui coast yesterday but could not be sure.
Orca travelled about 100-150km a day, she said, and it was common for orca to be seen close to shore.
There were fewer than 200 orca around the New Zealand coastline.
Dr Visser had not had many reports of orca near Wanganui but that wasn't to say they didn't happen.
She encouraged people to report any orca sightings to 0800 SEE ORCA.