Police can "only speculate" on how a seal ended up on a farm in central Waikato, which is around 80 kilometres from the Firth of Thames. 

The sea mammal was found in a dried-up ditch near Morrinsville yesterday morning.

"Our staff corralled the animal about 11am and awaited the arrival of Department of Conservation (DoC) staff who were able to return the female seal back to the ocean at Tapu about 5pm yesterday," said Sergeant Vic Sneddon, officer in charge of Morrinsville Police.

"We can only speculate, but we suspect during the recent heavy rains the seal made her way from the Firth of Thames down the Piako or Waihou Rivers via drains to where she was discovered between Morrinsville and Tatuanui."

Mr Sneddon said while the seal was far from its natural habitat it wasn't too worse for wear.

"We have images of her chasing one of our staff around a patrol car so while it may have had problems adjusting its internal GPS, she still had plenty of life left in her. But in terms of how she came to be there in the first place, its lips were definitely sealed.

"DoC Ranger, Garry Hickman, advised us that when last seen, the seal appeared to be swimming happily in the general direction of Waiheke Island."