"At the moment we don't know who is at fault in that aspect but our investigations are continuing," he said.
"There is a six-month statute of limitations, we don't have to find out who's done it in two days' time, so we are going to be methodical and work it through and hopefully someone might come forward."
A Kawhia local who did not want to be named branded the spill as "a form of eco-terrorism" and said she had to put off fishing for kahawai and snapper that night.
The woman said the person responsible "definitely isn't local" and she was upset the investigation could take longer than she thought.
"If they were [local] we would have known about it straight away," she said.
"It looks like the worst of it has gone, but it's pretty disgusting to think a person can show up here, dump a lot of fuel or whatever it was and then just bugger off, hoping they'd get away with it."
Mr Munro said there was still some concern over eel grass, shellfish beds and reeds at the high tide mark but he believed the damage to the eco-system was minimal to non-existent.
He said oil-absorbent booms were not required to help with the spill.
"We are not looking at extensive oiling at this point, it's more around the vicinity of the wharf about 300 metres from the entrance.
"The good thing with diesel is that any kind of agitation helps to break it down, so we are not expecting any long-term damage to the harbour system."
Mr Munro said the person responsible could face anything from a warning or fine to being charged in court.
