An 18-year-old Whanganui teenager who was involved in three separate aggravated robberies will probably spend his 21st birthday behind bars.

Tamehana Thompson and two co-offenders targeted BP 2go and Black Bull Liquor in Whanganui East two years ago.

Thompson was the driver on November 7, 2017 when his co-offenders entered the BP on Anzac Pde, one wielding a hammer and the other a knife.

Two store workers exited the building at the time, leaving the offenders to take cigarettes and cash worth $3700.

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Thompson drove his co-offenders away from the premises and just two days later on he drove them to Black Bull Liquor on Duncan St.

This time one was wielding a hammer and the other a steel bar when they confronted a staff member before stealing $2583 worth of tobacco, alcohol and cash.

Thompson was arrested and appeared in Whanganui District Court on November 21 when he pleaded not guilty to two charges of aggravated robbery.

He was granted bail by Judge Dugald Matheson but his offending did not stop and he was back at it again on September 20, 2018.

This time, Thompson and one co-offender targeted a dairy on Barriball St in New Plymouth.

At the Whanganui District Court on Monday Judge Philip Crayton said Thompson drove to the dairy with the intention of robbing it.

"The person who went into the store had a hood pulled up over their head, face partially covered, socks on their feet and white gloves. They had a knife similar to a kitchen knife.

"There was a carry bag with them and they went to the counter. Two female victims were standing behind it. The knife was brandished and a demand was made."

The offender jumped the counter and took cigarettes and tobacco valued at $8000.

Lawyer Roger Crowley said Thompson's offending was driven by his desire for approval and acceptance from his peers, something he had never had growing up.

"Tamehana Thompson never really had a chance. He was passed around like a chattel amongst his family, some of whom made better efforts than others to support him, to provide a stable environment and to show him love," he said.

"But it was like a merry-go-round and the cultural report in my submission is vitally important to the court. It doesn't excuse in any way the offending, but this really is a young person who deserves some mercy from the court."

Judge Crayton took Thompson's youth, his guilty plea, remorse shown and the cultural report into account when delivering his sentence.

Thompson was a product of his upbringing, he said.

"Unfortunately if we have not received support through childhood, it is little surprise that we seek support from those who will give it to us," he said.

"On occasions that means inevitably mixing with people who you consider are friends, but in fact are just going to lead you into trouble."

Judge Crayton sentenced Thompson to four years and three months' imprisonment.