It could be a trawler bot picked up your email address from a web site.
Or your contact info was sold to another vendor.
I did a google of your business name, nicollengineers.com.au
and got this staight away,
Nicoll Ross J & Associates - East Brisbane, QLD - whitepages.com.au
My partner has this problem too, and it comes back to the bots trawling over her business website, and either guessing email addresses, or finding an email link.
It's very frustrating, as your address or site can end up on a 'blacklist / banned site list', making it difficult to get legit business traffic.
You need a good spam filter at your ISP to stop most of the spam reaching you.