Authorities urge skippers to be aware of surroundings and maintain safe distances.

The reminder follows a significant increase in the number of whales migrating north through NSW waters and a serious incident involving a fishing boat on the South Coast.

Just after 9am on Sunday, June 6, Marine Rescue NSW received a MAYDAY call after a breaching whale landed on top of a recreational vessel off the coast of Narooma.

NSW Police said the 39-year-old male skipper advised that his 18-year-old male passenger had sustained a serious head injury and was trying to navigate back to the boat ramp despite the vessel taking on water.

NSW Ambulance paramedics met the vessel at the boat ramp and treated both men before taking them to hospital for treatment.

Police said the younger man was later airlifted from Moruya Hospital to Canberra Hospital, where he remains in a critical but stable condition, while the older man was treated at Moruya Hospital for facial lacerations and concussion.

Marine Area Commander, Superintendent Joe McNulty, said Maritime NSW has launched an investigation into the circumstances surrounding the incident.

"While inquiries are in their infancy, the incident demonstrates the dangers these mammals can pose to those on the water," Mr McNulty said.

"In recent days, the number of whales migrating north has dramatically increased, and maritime authorities have received reports they're travelling closer to the coast than in previous years.

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"Given the close proximity to the shoreline, there is potential for some spectacular whale watching, but we encourage anyone hoping to get a closer look to maintain a safe distance as outlined in the Biodiversity Conservation Regulation 2017."

More information about whale watching and safety tips from NPWS are available online.

Police believe the whale may have also been injured during the incident and NSW National Parks and Wildlife Service (NPWS) will work with Organisation for the Rescue and Research of Cetaceans in Australia (ORRCA) will monitor to ensure assistance is provided, if required.

Anyone who sees a stranded, entangled or distressed whale is urged to contact call NPWS on 1300 0 PARKS.

Mr McNulty said it was a timely reminder to keep safety at top of mind at all times and adhere to approach zones whether in, on or above the water.

"With tens of thousands of large whales currently migrating north, we expect large numbers of vessels to take to the water, all water craft (including surfboards), vessels, swimmers and spectators must adhere to regulations - not only for their safety, but also for the protection of these magnificent creatures," Mr McNulty said.

"You must not approach a whale any closer than 100m on a vessel, including boats, surf skis and kayaks, or 300m on a 'prohibited vessel', which includes jet skis.

"Swimmers and divers must not enter the water within 100m of a whale and then make sure they maintain a distance of at least 30m in any direction.

"For those lucky enough to view from above the water, unmanned aircraft (drones) must not encroach closer than 100m, while other aircraft must maintain a distance of 300m, except helicopters and gyrocopters, which must maintain a distance of 500m.

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"In addition, all the normal marine safety rules apply, which includes the skipper's responsibility for the safety and well-being of all on-board their vessel.

"This includes making sure the vessel is in good working order and carrying all the correct safety equipment, including a life jacket for every occupant.

"Most importantly, check the weather and marine conditions before you head out and while you're out to make sure you make it back to shore safely."

For more on-water safety information, CLICK HERE.