Barmaid Danielle Ormond suffered broken ribs and fractured vertebrae in her neck and back and worker Neil Coffield and customer James Guthrie were also injured in the blast at the Drumtochty Arms in Market Square, Auchenblae, on January 7, 2009.
Work was being carried out on the newly renovated premises' gas system at the time and Instant Catering Maintenance Ltd (ICM) was designing and installing a new ground floor kitchen with appliances fuelled by propane.
ICM employee Mr Coffield was purging the gas system when Ms Ormond noticed a strong smell of gas. The explosion happened just minutes later, causing a large part of the building to collapse.
Aberdeen Sheriff Court heard that the steel manifold to supply propane gas to appliances was not fitted with the correct equipment to prevent gas build-up to dangerous levels.
Regulators, which ensure correct pressure, had been fitted by another ICM employee the wrong way round and they had also been set for natural gas rather than propane gas. The chargrill had not been fitted with a regulator. ICM, of Union Street, Aberdeen, was fined £7,500 after pleading guilty to breaching a section of the Health and Safety at Work Act.
Health and Safety Executive Principal Inspector Niall Miller said: "This was an entirely avoidable, very serious incident."