such an event as

[navi tasan]

Which are correct:

1) If such an event as mentioned before occurs, the authorities will have to be notified.
2) If such an event as the one mentioned before occurs, the authorities will have to be notified.

3) If such an event as those mentioned before occurs, the authorities will have to be notified.

4) If such events as mentioned before occur, the authorities will have to be notified.
5) If such events as those mentioned before occur, the authorities will have to be notified.

"1" and "2" are supposed to mean the same.
"4" and "5" are supposed to mean the same.

Gratefully,
Navi.

 

[bhaisahab]

They are all overly wordy and unnatural in my opinion.

 

[Raymott]

"If that ever happens [again] ..."

 

[Tdol]

Without any more context, I would probably use 2, 3 & 5. They are a bit wordy, though.