The juvenile dolphin was lying prone at the edge of the mudflats during low tide at Blueskin Bay about 3pm, when spotted by a KiwiRail worker.
KiwiRail track ganger Matt Johnston said one of his workmates spotted the dolphin about 30m from the rail track, lying on the mudflats of Blueskin Bay.
The four-man crew went and inspected the mammal with the water about 600m away.
The workers started digging around the dolphin trying to get some water on it.
"We just tried to get it comfortable.
"You could tell it was still alive, it was moving and breathing, so we just kept working on it," he said.
Project Jonah spokeswoman Louisa Hawkes said the workers did what they were told.
"So they got water on the animal, they dug out around the fins, they monitored the breathing and let us know the breathing rates which helped us ascertain what kind of stress levels the animal was in.
"They followed the instructions really, really well," she said.
The workers were on their own for about 45 minutes before the cavalry arrived.
An assessment of the health of the dolphin was carried out with help from specialists at Massey University.
"We had to look at things like body condition and breathing rate, stress levels, and any indication of injuries," she said.
"So once we went through that process, the dolphin came out as a good candidate for a refloat."
It was then put into a dolphin lifting mat which gave it an opportunity to regain its balance. It quickly took off in the water and was seen heading out to sea.
The rescue took about four hours, with the dolphin heading out to sea about 7pm.
Ms Hawkes said the dolphin may have ended up in the bay as it was chasing a fish and came too far inland.
Mr Johnston said it was one of the more unusual days at work.
"It wasn’t something you do everyday and was a bit out of the routine. But we got a result in the end."
