I am not aware of any official advice from the Ministry of Health relating to co-sleepers (also referred to as bedside sleeper or "co-sleeping bassinet”). As far as I am aware there are no regulations relating to co-sleepers in New Zealand, although there are some in the US.
As you point out there are potential advantages of a co-sleeper. They achieve room sharing without direct bed sharing. In my opinion, co-sleepers are not as safe as bassinets (simply because more things can go wrong) but are nowhere near as dangerous as sharing a bed. They probably have about the same risk as wahakura or Pēpi-Pod.
The main risks to avoid are:
- Baby getting stuck between the two beds (entrapment). A co-sleeper should be securely fixed to adult bed to avoid a gap.
- Loose bedding from the adult bed
- Bed height not aligned perfectly. If the top rail of the co-sleeper is higher than the adult bed surface, there’s a risk of the baby’s neck getting caught onto it.
On the other hand, a lower sleeping surface increases the risk of some of the bedding sliding over and down to their bed.
A study from the US found a total of 26 incidents (6 deaths and 20 injuries) were reported to the Consumer Product Safety Commission. Of these, 5 deaths were caused by asphyxia, and 1 was attributed to SIDS. Almost half of the injuries occurred after the co-sleeper was improperly assembled. This suggests death and injuries are infrequent.
Note. My only other concern is the cost. A quick search online suggests prices around $300+ in New Zealand, which is only safe to use for a few months (US regulations state up to a maximum of 5 months). For many families, the cost might be better spent on a normal cot which will last many more months.