A convicted murderer has added one extra week onto his life imprisonment sentence for having a sim card in prison with him.

Anthony Paul Doyle was jailed in 2006 for shooting dead Mikaere O'Sullivan and Toni-Anne Nathan under a bridge near Tauranga the year before.

Doyle shot the Omokoroa couple when they met under the Wairoa Bridge to settle a drug debt.

He was given two life sentences, with a minimum non-parole period of 19 years.

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Doyle, who is currently serving his sentence in Whanganui Prison, appeared in the Whanganui District Court on Tuesday on one charge of breaching the Corrections Act.

Corrections staff discovered a sim card in his cell on December 9, Judge David Cameron said.

Doyle, who was represented by Anna Brosnahan, pleaded guilty to the charge.

Judge Cameron sentenced him to one week in prison, cumulative on his existing sentence.

Also in court for breaching the Corrections Act was Alexander Robert Coe.

Coe had "an unauthorised communications device" on December 23, Judge Cameron said.

He was heard talking on a cellphone by a prison patrol.

"These are prohibited items and the reason is for the security of the entire prison community. Prisoners are not allowed to possess cellphones. That would give them freedom to deal freely with members outside the prison."

Coe also pleaded guilty and received a week in prison on top of the sentence he was serving.

Tamati Ruka Thomas also pleaded guilty on Tuesday to breaching the Corrections Act after he was found in Whanganui Prison with a cellphone in November.

Thomas is serving a sentence for injuring with intent to injure and wounding with intent to cause grievous bodily harm.

Judge Cameron said prisoners possessing cellphones "undermines the security of the prison".

Thomas also received a week in prison, cumulative on his existing sentence.