There are a few jobs where any criminal conviction bars you from employment. (Some police forces, for example, might operate this policy but others, like the Metropolitan Police, don't automatically reject people with minor convictions). Job descriptions vary greatly under the general umbrella of 'civil service' jobs. As long as your conviction doesn't automatically bar you from employment (and you explain your misunderstanding of the definition of a 'criminal offence' to your employer), it's unlikely that your conviction will damage your immediate employment prospects.
Offences which are dealt with by fines are usually 'spent' after 5 years (or half that time for someone who is under 18 on the date of conviction). So, for most types of employment you won't have to declare this conviction after 5 years. (If the particular area, in which you work, is exempt from the provisions of the RoO Act, you'll have to continue declaring the conviction throughout your career but it's unlikely to carry any great weight when your applications for advancement are considered).
A couple of final points:
1. Although I doubt that your conviction will affect your career, an employer who already knows about an offence is not obliged to disregard it when it becomes spent.
2. Remember that you must disclose this conviction, when required to do so, for 5 years. Failure to do so, whether it's applying for another job or simply filling in an insurance application form, will render you liable to prosecution for 'seeking to gain a pecuniary advantage by deception'
Chris