How to hire SEO assistance

If you mean, hiring people there's a number of reputable SEOs from this forum who would be willing to take the job on. You can see a list of our members, filtered by reputation here: https://seochat.io/members - drop whoever you think may be suitable a message.

As for LEARNING SEO, I always think a nice place to start is Moz' free beginner's course in SEO: https://moz.com/beginners-guide-to-seo

What kind of traffic decline are you seeing?

Gradual over time or a drop at a very specific time?

Is it all from one source i.e Google or every source?

I recommend checking out Google Search console and looking at the manual actions and security messages.

Try over at SavvySME. You’ll find a lot of Australian-based SEO experts in one place, which searches so much easier 

Depends on what spathes you're talking about. There is always a minimum swing