The radiation levels are more than double the previous record high that was reached in early June.
One nuclear expert predicts the clean-up from the March 11 earthquake and tsunami will be even more difficult, but there is speculation that the reading could be an aberration.
Peter Burns, former chief executive officer of the Australian Radiation Protection and Nuclear Safety Agency, says given the scale of the Fukushima emergency, the high reading is to be expected.
"The levels reported of 10 sieverts per hour are very high levels and it's going to be very difficult to manage workers going into those areas and doing operations," he said.
"To put the 10 sieverts into context, that 10 sieverts is actually a lethal dose of radiation. So you can't afford to be exposed for more than a few minutes at those levels.
"It means you're directly exposed to fuel rods in the reactors or the spent fuel ponds very closely and while it's possible to get to those levels it means there is very little shielding going on there."
In the week after the quake and tsunami, which triggered an explosion at the facility and a nuclear meltdown, military helicopters water bombed the power plant to try stop fuel rods and containment pools being exposed to the air.
Mr Burns says in addition to the damage, those working on the reactor now also have to contend with contaminated waste generated by the clean-up operation.
"There have been reports it's a huge problem of a huge inventory of contaminated materials - water and other materials that are going to have to be managed over the next years," he said.
Tony Irwin, a former reactor manager now with the Australian National University, says TEPCO has now set up a water treatment plant.
"To pump this contaminated water through this treatment plant, you remove the radioactive materials and this is the water that is now re-circulating back to cool the reactor," he said.
"So all this water is now being contained and treated."