Question: how to convert qnh to pressure altitude?
Answer: Brisbane's current QNH is 1023. In the International Standard Atmosphere(ISA) there is a 1 hPa difference for each 30 feet vertical change in height in the lower levels. 10 hPa * 30 = 300ft height variation. Because our QNH is above 1013 we have more air pushing down on us causing a higher local pressure.