Ive recently had my 2nd story addition sheeted with gyprock, the ceiling has sheets running across the battens (not parallel), but when i was in the ceiling i noticed it wasnt back blocked, is this an issue?

They have only blocked the butt joins...

My roof is steel with steel battens at 600 centres.

I will post a photo tonight when I get home, im looking to plaster the lot on wednesday so if they need to be backblocked I would need to do it before then.

thanks!

Correction, they are screwed off at 450 centres, not 600

Backblocking in this case, would normally be done only on the butt joins - as you've noted. If there were other factors like high humidity or fire control, it could be different.