PE piping systems shall be installed in accordance with AS/NZ 2033 or the manufacturers instructions. Later I will also attach the Guide for Safety with Underground services you can also look at G10 here on the forum good info there as well.

AS3500 refers to such cases being a subject to the engineers approval.

AS/NZS 5601.1 table 4.1 no PE pipe over 7kpa to be laid in the ground under the building

Fairly sure that if the pipeline in pe is run within a conduit, it can go under the floor slab. Think of the thousands of installations with PE that run under driveways and slabs etc....Best bet, call the gas board, they'll set you straight and you wont have to worry. Make sure you get them to send a letter in writing confirming the course of action to take and file it in with the paperwork of the job.