Why the entry exists at all
When land is first registered, the registry records what the deeds show. Where a conveyance referred to covenants but no copy could be produced, the register notes that the land is subject to such covenants as were imposed and remain enforceable — without saying what they were.
The same applies to rights granted the other way: a neighbour's right of way, a right to run pipes beneath the land. Those may exist too, and are equally unknowable.
Entries of this kind are common on older titles, particularly where documents were destroyed during the war or lost in the ordinary way over a century.
What can actually be found out
Sometimes more than you would think. Neighbouring titles were often created out of the same original conveyance, and their registers may quote covenants the lost deed imposed — because their copies survived.
Local archives, the registry's own historic records and the seller's own papers are all worth a look where the stakes justify it.
Often nothing. At that point the question stops being what the covenants say and becomes how the risk is handled, which is the subject of the companion page in this series.
The practical risk, and the thing not to do
In practice, unknown covenants are rarely enforced: somebody would have to know they exist, establish that they have the benefit, and show what the covenant required. That is a demanding chain and it is why these entries are usually dealt with by insurance rather than investigation.
The risk is not zero, and it is concentrated where you intend to do something — build, extend, change the use. A covenant that is never breached is never enforced.
The thing not to do is go looking in a way that alerts a potential beneficiary. Asking a neighbour whether they know what the 1923 conveyance said is exactly the enquiry that makes indemnity cover unavailable. Decide whether you are insuring or investigating before you start, because you cannot do both.