I'm unsure whether to hire a water tanker or just fill from the tap, we have water restrictions but I don't know exactly what they say regarding swimming pool filling

You call the water company and ask him,
Locally here, you can do it by tanker, you can even rent from the water department equipment to fill it form a fire hydrant ( they have special meters that fit on them) or do it from your garden hose ( if you have a couple days to wait to fill it )

So call the water dept and ask them options.