Those with bookings can still use the track and toilets, but it will be closed to day walkers. Photo: Supplied

A popular South Island alpine track has been temporarily closed to day trekkers because the toilets at one of its huts are almost full.

There have been more trampers than anticipated on the Mueller Hut Route at Aoraki Mount Cook National Park this summer.

The Department of Conservation (DOC) estimates the track is seeing up to 250 trampers a day.

Those with bookings can still use the track and toilets, but it will be closed to day walkers.

DOC Aoraki Mount Cook operations manager Sally Jones said the decision comes in light of protecting the environment.

"We are closing the track to day walkers because it goes through a pristine and fragile alpine environment, which will be negatively affected if people are toileting outside of the provided facilities. It is also a popular track and people toileting could pose a health risk to visitors.

"As there is still some capacity in the tanks, we are leaving the track and hut open to visitors who have booked their stay in advance."

With the hut largely booked out through summer, Jones was unable to say when the track would reopen to day walkers.

"Exact timing will depend on when the required specialist equipment is available.

"As there is still some capacity in the tanks, we are leaving the track and hut open to visitors who have booked there stay in advance. The hut is largely booked out through summer.

"We would encourage people to make use of the range of other tracks in the area which offer great alpine experiences such as Red Tarns Track and Hooker Track. More experienced hikers may want to visit Sefton Biv."