They watched the killer whales move quickly through the water, sometimes breaching and slapping their big paddle-shaped flippers.
"They just seemed to be playing a bit," Rogers said.
DoC records all orca sightings, and Treadgold was able to zoom in for close-up photographs.
Orca sightings on the Whanganui coast are relatively common, senior ranger/supervisor recreation and historic Jim Campbell said.
The marine mammals are actually a large dolphin rather than a whale, according to the DoC website. They feed on all kinds of fish, rays, squid, octopus and seabirds - and will even attack and eat seals and whales.
There is no record of them making fatal attacks on people.
A lot of kahawai have been running in the Whanganui River, Douglas said, which may have attracted them. But Campbell said they were most probably after stingray.
The pod consisted of an adult male, two adult females and two juveniles. Adult males can grow to 9m long. Females are smaller.
The females can live to 80 or 90 years old, and have about five young during their 25-year breeding lives. Young orca are nursed for at least a year.
The females and juveniles left, and the male is expected to follow.
The vertical dorsal fins and black and white markings of orca make them easy to identify. They usually prefer deep water, but sometimes come into shallow bays and estuaries.
"Pods of orca are known to venture into Wellington Harbour throughout spring and summer. Orcas in other parts of the world aren't known to do this, and it is rare for them to stick around," Campbell said.
There are an estimated 150-200 orca in New Zealand waters.
Boats are one of the biggest threats to orca. Boats coming near them are asked to idle, get no closer than 300m, and to approach them in parallel and slightly from the rear. They are not to circle them, cut through groups or obstruct them.
Swimming within 100m of orca is banned. Sightings can be reported by phoning 0800 DOC HOT (0800 362 4680).
