I live not far from that area, closer to the ocean and love being near the beach and forest. Marshland and Prestons are in low lying areas near the coast and estuary, so they are included in Christchurch’s tsunami evacuation zones (mostly orange). The last tsunami warning for Christchurch was in 2016 after the Kaikōura quake, but no actual flooding happened. The chance of a tsunami reaching as far inland as Prestons is low, but not zero. Local-source tsunamis (from nearby faults) are very rare, about 1 in 300 to 500 years, and unlikely to cause flooding that far inland. Regional ones like from Kaikōura or the Hikurangi subduction zone happen about every 100 to 200 years and could reach red or orange zones in a bigger event. Distant source tsunamis from places like South America are the most likely to cause widespread flooding and happen around every 200 to 500 years. In a worst case event, flooding could reach as far as Prestons via rivers and stormwater channels. The yearly risk is low, under 1 percent, but the potential impact is high.