Department of Conservation (Doc) booking services manager Cameron Hyland said the crash was caused by "extremely high traffic", as 10,000 people online tried to book one of the 7500 spots over the season.
By comparison, in previous years, there had been up to 7000 people online at opening.
The system seemed to recover just before 10.30am and Doc started to receive high volumes of bookings; however, there were still some issues causing booking frustrations, Mr Hyland said.
The matter was fully resolved about 1.15pm.
Mr Hyland acknowledged the difficulties it caused people trying to book a spot and said the department was working with its IT vendor to understand the cause of the issue, and how it could be prevented in future, as more Great Walks were opening soon.
"We have had the system crash before, for example in 2021 due to high volume and limited server capacity, but in this case we had enough server capacity."
The department had received some feedback asking why it did not shut bookings down completely during the crash, but in the past similar issues had been "self-resolving", he said.
"Once the initial outage period passed [yesterday], we were receiving high numbers of successful bookings and at that point, shutting down the system would have more negatively impacted those people.
"The Milford Track did sell out quickly despite the issues — though this was definitely a more frustrating process for the users than it should have been."
For those who missed out, inventory would be released in coming weeks as cancellations and modifications were made, so people would "see spots open up for some dates occasionally", if they checked at the right time.
Next Wednesday, bookings open for the Abel Tasman Coast Track, Kepler Track, Rakiura Track and Whanganui Journey, and next Thursday, bookings open for the Paparoa Track, Routeburn Track and Tongariro Northern Circuit.
On May 16, the Heaphy Track can be booked from either end up to 19 October, the full track expected to be open later in the year.
