Federal Environment Minister Greg Hunt has given approval for "ecological thinning" trials in a new national park on the Murray River, a move green groups say marks the first time logging has been permitted in forests granted the highest level of protection.

At the end of last month, the Environment department approved with conditions selective logging over five years in 44 plots within the Murray Valley National Park in NSW.

Murray River red gums: Logging will be permitted over five years in 44 plots within the Murray Valley National Park. Credit:Karl Quinn

The previous Victorian coalition government had earlier pulled out of the trials in their state's side of the Barmah-Millewa forest, the largest stands of river red gums in the country.

The NSW government has argued that the viability of high-density stands of red gums is at risk during extended droughts, citing research done towards the end of the dry spell in 2009 – just prior to the creation of the national park the following year.