Hi Xin,
According to the New Zealand Flora Series Vol. IIII (Healy & Edgar 1980) Trachycarpus fortunei was first recorded naturalised here in 1959 - however, all that date provides is a rough yardstick because I learned some years ago that what the Flora accepts as a "first record" is not necessarily so, e.g., Banksia integrifolia is recorded as first naturalizing here in 1991, when in fact the first record of this occurring wild in a N.Z. herbarium was a Rotorua collection made in 1987. I see you are based in Auckland, so for control measures why not ring the council? They have staff versed in plant and animal pest control.
Ciao
Peter
Healy, A. J.; Edgar, E. 1980: Flora of New Zealand. Vol. III. Adventive Cyperaceous, Petalous and Spathaceous Monocotyledons.Wellington, Government Printer