The error being thrown was:
ORA-01840: input value not long enough for date format
01840. 00000 - "input value not long enough for date format"
That's fairly straight forward, it just means that the date I am trying to convert doesn't have enough precision for the format that I am trying to convert it to. The SQL below demonstrates this...
select to_date('2016', 'YYYYMMDDHH24MISS')
Interestingly the error disappears when I add a month and day to my date value i.e. 20160101 is OK even though my format expects hours, minutes and seconds too.
My workaround takes advantage of the above find by padding the date up to 8 characters (yyyymmdd) as required. It consists of two rpad operations, one for the day and one for the month to cater for inputs of the 'yyyy' and 'yyyymm' formats.
This is the end result...
select to_date(rpad(rpad('2016', 6, '01'), 8, '01'), 'YYYYMMDDHH24MISS')
It's not the best solution but will get you out of a bind if you really need to pad dates like this.