If the hive is strong, you can open it. If it's not strong, you can open it but keep an eye on things, particularly if there's no flow on. if robbing occurs shut it down to a single bee width - they can defend that
If you're in New Zealand (as your flag suggests) your season will be completely different to ours.
Personally, I don't use entrance blocks over winter but I do use fine wire mesh that allows ventilation but restricts the entrance to prevent intrusion by robbers/etc. In summer, they are wide open as the colony is more than strong enough to defend itself
Entrance block removed last night and bees inmmediately seemed happier. They had been squeezing through the letterbox and now the front door's wide open!