People have reported problems booking huts and campsites on the Milford and Lake Waikaremoana Great Walks for this summer.

Disappointed people took to social media to vent their frustration on Tuesday.

DOC has apologised to trampers and says there were 11,000 people logged in waiting for bookings to open at 9.30am.

The system was struggling so it decided to do a reset that would have looked like a crash to some people, it said

The Department of Conservation's website crashed last year when 10,000 people from New Zealand and overseas tried to book a walk on the Milford Track.

The site came back online shortly after and the 7500 spaces sold out within hours, leaving thousands disappointed and without a spot.

DOC had earlier provided assurances that last year's issues would not be repeated.

New Zealand's 10 Great Walks are the country's premier and most popular tramping tracks. Every year thousands of people from New Zealand and abroad try to secure a space on one of the tracks.

Federated Mountain Clubs president Megan DiMozantos said she was not confident DOC would get it right this year.

A lot of time and money went into planning Great Walks hikes and bookings should be allocated on a "Kiwi-first" basis, so people did not miss out, she said.

"We believe... New Zealanders should get first dibs on the Great Walk bookings once they open before they're opened to internationals and to commercial," she said.

Gerard Hindmarsh - an avid tramper with a lifelong interest in New Zealand's back country - said the booking system was frustrating and impossible to navigate.

People struggled to secure dates and thousands missed out each year, he said.

"It's a hectic affair and there's thousands of Kiwi trampers that are still left extremely disappointed that they can't get their dates and it's impossible to often book long weekends or school holidays on some of our most popular tracks, which sell out completely," he said.

Having access to the tracks was "New Zealand's legacy" and opening up the booking system to residents three months before the rest of the world would ensure locals did not miss out, Hindmarsh said.

DOC director of heritage and visitors Cat Wilson said more Kiwis than ever before were hiking the Great Walks after the department introduced cheaper pricing for New Zealanders in 2018 to better manage international demand.

Differential pricing and opening bookings at times better suited for New Zealanders had immediately increased the number of locals on the tracks.

"Even with international visitors returning over the past year, the Great Walks continue to attract high levels of domestic tourism," Wilson said.

"Previously New Zealanders made up 40 percent of visitors, whereas the year-to-date mix is about 63 percent New Zealanders and 37 percent international visitors."

The department had made changes to the website and tested it ahead of bookings for Milford and Lake Waikaremoana Track opening Tuesday morning.

Wilson would not elaborate on what those changes were, but said she was confident there would not be a repeat of last year.

Main image (RNZ/Tess Brunton): The system for booking slots on the Great Walks has been plagued by crashes in recent years.