US Ambassador to Japan Caroline Kennedy looks around the central control room for the unit one and unit two reactors of the tsunami-crippled Tepco. Reuters

The operator of the Fukushima nuclear plant has started pumping groundwater considered to be within legal radiation safety limits into the Pacific Ocean.

Tepco said it has started diverting groundwater into the ocean in a bid to reduce the volume of contaminated water.

The diversion plan is an attempt to prevent unpolluted groundwater from flowing under the plant, and mixing with water contaminated with radioactive isotopes.

Fukushima operators have been struggling with water contamination since the plant meltdown in 2011, caused by the earthquake and tsunami that devastated parts of Japan.

According to AFP, coping with the huge amounts of contaminated water has been the biggest challenge for Tepco. It is estimated up to 400 tonnes of groundwater is contaminated beneath the site every day.

Water well unit haul from subsurface water is seen near No.3 (L) and No.4 reactor buildings at Fukushima nuclear plant. Reuters

After passing safety tests and following an agreement with fishermen, engineers started releasing water into the Pacific.

A spokesman said: "With the bypass system, we are able to reduce [the contaminated water] by up to 80 tons per day."

Kazue Suzuki, a campaigner for Greenpeace Japan, said the decision for the fishermen to agree to the pumping "must have been agonising", but that they had no other option.

At present there is no solution for dealing with the thousands of gallons of contaminated water stored at the site, although many experts suggest it will eventually have to be released into the sea.

The announcement comes after one of the decontamination systems at Fukushima was shut down following a fault.

Tepco said its Advanced Liquid Processing System was placed on standby after processed water was found to be cloudy instead of clear, Japan Today reports.

Higher levels of calcium were thought to be the cause, but why the levels were raised is unknown.