The Evander, believed to be an 8m-9m former commercial fishing boat, was reported to have 500 litres of diesel on board, Environment Southland oil spill response on-scene regional commander Dallas Bradley said.
Late yesterday, divers had plugged the fuel tank breathers and were working out how to refloat the boat.
Riverton Fire Brigade chief George Seator said the brigade had been down to see if they could assist with lifting the boat but were unable to help, as it was on the bottom.
"It's now in the hands of the owners and their assessors."
Mr Bradley said high spring tides and strong currents meant staff were unable to prevent the diesel from moving upstream into the Aparima estuary early yesterday.
While a patchy slick of diesel was reportedly visible at the town bridge in the early afternoon, a weather change had helped disperse the oil by later in the day, he said.
"There is no sign of any diesel [now]," he said.
Earlier there had also been concerns about its impact on spawning whitebait, but as of late yesterday there had been no report of any environmental damage, he said.
Mr Bradley said he did not have any information about what caused the Evander to sink, but there were no indications that it had any connection to surging water levels related to the tsunami on Sunday.
