Planning is already well under way for the rise in the falls' popularity.

More than $10 million is being spent upgrade the Nitmiluk Gorge's walking trails, lookouts, jetty and visitor's centre and Leliyn is next in line.

Leliyn is about 45 minutes by car north of Katherine.

More than 260,000 people visit the gorge each year, which about the total of the NT's population and makes it the third most popular tourist attraction in the Territory.

The falls welcomed 132,000 visitors last year and that number has quickly risen from 97,000 in 2014.

The NT Government has developed a "masterplan" for Leliyn and one of the first steps is to upgrade the park's infrastructure.

A tender has just been released to boost the capacity of the park's sewerage systems, which are now about 20 years old, and need to be bigger to cater for expanded caravan parks and camping grounds which are planned.

The new wastewater works are expected to cost about $1 million.

Darwin consultancy, Byrne Consultants, produced a report for the Government on the adequacy of the park's wastewater and found it lacking.

It is no surprise the consultants found most of the tourists (and locals) visited between April to October, or the dry season with the peak month in July.

The least number of tourists arrive in February, the wet season.

They calculate the 350,000 boom by 2039, the consultants used the average five per cent per annum growth factor at Leliyn.

At its July peak in 2039, the park can expect 2366 visitors each day, the consultants say.

The consultants say the number of overnight campers is currently restricted by the number of current campsites (53) at Leliyn.

Key extra facilities proposed in the "masterplan" include The key additional facilities outlined in the Master Plan include:

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