VOC assessments measure and test against the standard(s) that industry is currently using within the workplace;
As an example, if you hold a qualification for track dozer operation RIIMPO308E then the VOC assessment would measure you to that same standard.
If you hold an “in-house” assessment certificate from your previous employer that does not have any formal or national qualification standard, then the VOC assessment will not have a recognisable standard to measure against.
Whilst the in-house standard you currently have may be considered by some to be OK, the VOC assessment will not recognise an “in-house” standard and therefore you should complete training and assessment to a nationally recognised standard prior to completing the VOC assessment!