Working out pay for SLC

I travelled to Australia 3 months ago to work on a working visa.

I'm trying to fill the form out for student finance Wales to tell them that I've left the UK for more than 3 months.

The form is asking me for my gross annual pay before tax but I'm on a casual employment basis where I can work 6 days a week or 1 day a week.

How do I calculate for the form? I don't want to get caught out and have to pay extra.

Thanks for any advice.