The resulting fire ignited vegetation on a nearby cliff and swept up the side of a historic pa at Rangihoua Heritage Park, the site of New Zealand's first European settlement.
About 30 firefighters with seven fire appliances responded to the blaze, a helicopter finishing the job the next morning.
Moffat told court he did not believe he had been risking starting a fire, so he could not be criminally liable for arson. If he had thought there was a risk, he would have buried the bomb 50m further down the beach.
He said he added the petrol to get a bigger bang, and because it was cheaper than solid explosives, not because he wanted to start a fire.
Police argued Moffat's friends had alerted him to the bomb's proximity to cliffside vegetation, and to get to the beach he had driven or walked past at least half a dozen 'No Fire' signs.
Police appealed the District Court's decision to discharge him without conviction on the grounds the sentencing judge erred in his assessment of the gravity of the offending by overlooking relevant considerations.
The Judge overlooked that Moffat was put on notice by his associates that the hole he dug and into which he put the explosive device was too close to vegetation, that there was a total fire ban in place at the relevant time, and that he was familiar with and held pyrotechnic certification and experience, police argued.
Further, police said the sentencing judge erred in his assessment of the consequences of a conviction such as on Moffat's job, his family business, ability to travel overseas and obtain mortgages, that were unsupported by available evidence.
"Mr Moffat's actions were more than naïve as the Judge suggested. They were also much more than "a stupid thing to do", again as the Judge suggested," Justice Edwin Wylie noted in his judgment.
