I'm travelling to Sydney from Brisbane on Business Class on Qantas for the first time(upgraded via points, class U).

I'm travelling to Sydney from Brisbane on Business Class on Qantas for the first time(upgraded via points, class U). I am only on the lowest tier, Bronze. Am I still eligible to use the Business Lounge? Qantas website is not clear.

Yes, you will be eligible to use the Business lounge before departure. 

Absolutely! It's a very nice lounge, too - if you're NB it's worth nipping in a little early to enjoy it.

Sweet, thanks guys, I'm fully intending on enjoying every bit of it, leaving work early to get there well in advance.

If it's after work on a weeknight, you might be lucky and have island dining in addition to the buffet.

Don't forget that the the Business Lounge is the inner area of the Qantas Club domestic Lounge, and only those flying Business class that day are permitted into that area.  It is generally quieter, and the food offerings are slightly better.  It doesn't matter what your QFF status is, and you don't even have to be a member of the QFF, for your Business class boarding pass gives you the entitlement.

I think the only location where the business lounge is inside of the Qantas Club is in Melbourne - Sydney's business lounge is a seperate entry to the Qantas Club

The OP is using the SYD lounge, which has a separate entrance. IIRC the Business Lounge entrance is to the left of the main security checkpoint, before the Qantas Club.

It's easy to miss it and end up at the QP - I did that at 0530 in the morning once as an upgraded NB, and had to be directed back to the Business Lounge (which is down a corridor past the lounge dragon, if you're already upstairs).

If I check in using the app, and my boarding pass is in my passbook on my iphone, that counts? Or should I print out a boarding pass just to be on the safe side?

You'll be fine if you do that, as your travel class will show up on the mobile BP. However, you can always print out an OLCI boarding pass if you prefer - and you can even keep the PDF to remember your successful upgrade!

Yes thats fine, no need to print a boarding pass.

The OP states travel is to Sydney from Brisbane.

To reach the Brisbane business lounge you enter the main lounge entrance which is common to both the Qantas Club and business class lounge. The entrance to the business lounge is to the RIGHT of the service desk immediately through the glass doors - there is then a subsidiary doorway and a second desk on entering the business lounge.

You will most probably get even more time to enjoy.So many delays in and out of Brisbane.Have to love a modern airport with one usable runway for flights.

You can invite a guest too - even if they aren't travelling anywhere ;-)