Emergency response staff are responding to what appears to be a large diesel spill in the lower reaches of the Whanganui River.
The spill was reported to the Whanganui Manawatu regional council this morning.
Emergency management manager Ian Lowe said the spill was affecting a large area of water between the river mouth and the boat ramp.
"At this point the source of the spill is unknown. However, we can confirm it is not as a result of river management work underway for the Te Pūwaha port revitalisation project at the North Mole.
"Our compliance team are working to determine the source of the spill. At this stage we don't believe it to be linked to any boats in the port either."
Horizons Regional Council was pulling out the stops to respond to the spill, Lowe said.
"We are mobilising equipment and more staff from Palmerston North to assist with the response. This will include a boat to access the extent of the spill and possible source."
The first priority was to contain the diesel and remove as much as possible from the awa.
"We will put inflatable booms out to stop the diesel from spreading out further and use absorbent pads to soak up what we can. There is currently an incoming tide which we will use to our advantage for containment."
Lowe said the effects of the spill could be long lasting.
"While we will do our best, it's unlikely that we will be able to absorb all the diesel and expect any elusive patches will evaporate overtime. People are likely to see lingering effects of the spill and Horizons will be monitoring the situation over the next few days alongside and under the leadership of hapu."
Lowe said people in the port vicinity would see staff undertaking the work most of today.
Local iwi have been notified of the event.