However ACC (Accident Compensation Corporation) says her outcome was not affected by the delay in diagnosing the tumour.
ACC spokeswoman Stephanie Melville said the corporation acknowledged the failure to provide adequate treatment, but Mrs Moult did not sustain a personal injury as a result.
In reaching the decision, clinical records were considered along with external clinical advice from a GP, emergency medicine specialist, neurologist and neurosurgeon.
ACC found the care provided by Mrs Moult's GP in May 2011 was appropriate.
"After considering the clinical evidence and opinions, we found there was a failure to provide adequate treatment when Mrs Moult presented to the emergency department in July 2011."
"However, the evidence and opinion did not support that Mrs Moult sustained a personal injury caused by treatment failure."
"Mrs Moult's tumour was a very slow-growing one and the delay from July 2011 to when the correct diagnosis was made in August 2011 did not alter the outcome had such treatment failure not happened."
Mrs Moult said the rapid progression of her symptoms did not indicate a slow-growing tumour.
She said her numbness and loss of sensation emerged only a few weeks before diagnosis, and were not apparent when she first presented at Middlemore.
"I wouldn't have had loss of sensation completely on one side for the rest of my life if they spotted it earlier."
Counties Manukau District Health Board has acknowledged Middlemore's emergency department failed to provide an adequate standard of care, and has apologised. The board also refunded the cost of her MRI.
Mrs Moult will also make a complaint to the Health and Disability Commissioner.