The hole was 16 meters long and up to 4 meters high in August 2017. It has doubled in size since the discovery in October 2016 when experts told Auckland Transport (AT) "it should be looked at immediately".
According to AT chief executive David Warburton there was no suggestion of imminent failure or public safety risk due to the hole.
AT started design work to repair damage and intends to begin work in October and complete the job before summer holidays. The repairs are expected to cost less than AU$1 million.