Mr Trafford said it took out two notable pine trees, leaving other big trees now facing the risk of being lost by continued erosion caused by the stream along the base of the bank.
Residents estimate that about 20m of this section of the park has been lost from erosion over the last few decades.
Mr Trafford said there was not much the council could do to stop further slips because it was caused by the stream eroding the base of the bank.
The council responded quickly yesterday by fencing off the slip, which has brought the bank closer to the loop road.
"There have been a number of slippages there over the last five years but this one is the biggest," Mr Trafford said.
Many of the 40 recently planted pohutukawa, put there to assist with the stability of the bank, were lost in the slip.
Mr Trafford said the three or four big trees now close to the new bank would be pruned to reduce their weight on the bank.
Hopefully the reduced weight would slow down the period between slips although it was unlikely to stop them happening.
"There is not much we can do to stop it," he said. The big trees were some of the original ones planted there and may have been part of a shelter belt in the days when the area was a farm.
The slip has left a steep drop and the council has urged visitors to the park to be careful around this area, and to keep behind the fence. The trees threatened by future slips are a mixture of pines and macrocarpa.
