A man who hid a hypodermic syringe in a Wanganui supermarket and claimed there were another nine or 10 concealed in the store has failed to get a reduction in his 16-month jail sentence.

Kevin Noel Field had earlier pleaded guilty to demanding with intent to steal, which carries a maximum of seven years' imprisonment.

His lawyer told the High Court in Wellington that the term was manifestly excessive but Justice Andrew Dobson upheld the sentence imposed by the lower court.

Justice Dobson said that a male caller phoned the duty manager on November 16 last year and told her to look in the bread room and that there was something for her under a crate.

The caller said there was a jet plane lolly under a bread crate and told the manager to touch it.

When the manager picked up the piece of confectionery, she saw a hypodermic needle under it, though she did not realise at the time that it was in fact poking through.

She was told that there were another nine or 10 such syringes around the store.

The caller said she had to put cash in a bag and put it in a rubbish bin outside a particular dairy.

The manager called the police, despite being told not to by the caller.

They identified Field who was seen on the video footage going to an area where bread was displayed and placing an item under a set of trays.

Field partially admitted the facts when spoken to by the police but said he could not remember due to his previous drug dependency.

The district court judge had said that Field had tried to extract money and the potential risk to the public and staff from the syringes was "very high".

He said that someone could have been stabbed and then faced the risk of contracting a disease.

The potential harm was "enormous".

Field was described as having a "bad record of criminal offending".

Crown counsel said that Field's conduct was calculated to play on the fear of contamination.

It took four hours to establish that other potentially contaminated needles had not been hidden elsewhere in the store.

Justice Dobson said that the case was made serious by the "insidious doubt" as to the extent of the risk.

"Were the needles contaminated? Had any contamination spread beyond the needles themselves? Where were the remaining needles?" he said.

Justice Dobson said that the starting point in the district court could not be regarded as manifestly excessive and Field was given a third discount for his guilty plea.