A New South Wales accountant who murdered a lotto winner because she could not control her spending has been jailed for up to 18 years.

Peter Kelly, 56, managed the finances of his friends Maria and David Devrell for 20 years.

The Supreme Court heard that after the couple won $5 million in Oz Lotto, the accountant argued with Ms Devrell about her excessive spending.

The court heard that in her Tamworth home last year, Ms Devrell pushed the accountant when he refused to give her more money.

Kelly said he then hit her over the head with a rubber mallet.

He pleaded guilty to murder after her body was found in a pool of blood.

Justice Robert Hulme rejected Kelly's argument that he was provoked.

He will be eligible for parole in 2024.