Being from the UK I can only really comment on what visas are available there.
My understanding is that visa availability varies considerably between different Countries & even between different Consulates & Embassies with any single country.
In order for you to discover EXACTLY what visas are available to you in Australia it is essential that you check with the web site (or by phone) of the consulate that you are applying to.
I say the above because I have never heard of a double entry Non Imm 'o' visa & I do not believe that such a thing is available in the UK.
A multiple entry Non Imm 'o' visa is available however. That permits individual stays of no more than 90 days but as many of those stays as you like for the life of the visa.
The visa will last for 1 year from date of issue (not date of first entry).
I have also been given double entry Tourist visas from the Thai Embassy in Vientiane. That permits 2 stays of 60 days within the life of the visa - which has a 3 month lifetime from date of issue.
So you need to contact your visa provider or their web site to see what is available to you.
Once in possession of a multiple entry visa though they all work the same as far as enacting the next piece of it.
& so, to answer your questions..
So I arrive in Nong Kai and cross over the bridge and then turn around and come straight back again..???
Yes, there is no minimum requirement to stay in Laos.
However, personally, as I would have paid the 30 USD (+ in my case as I am a Brit) for a month long Laos visa I would at least make the effort to go to Vientiane & have a look around, if not for a couple of days. You can get a taxi into the capital for 200 baht, or the bus for less, & it is well worth at least 1 look.
When do I have to do this ...??? On the day my 90 days stay runs out or before..??
The 90th. day maximizes your stay, yes. But you can go out any day up to that date without issues.
Also, when I come back again, this will give me a further 90 days...???
Yes. Remember to put your existing visa number on the arrival form & check before you proceed though...
So should I book my return flight ( back to Australia) before the 180 days just to be on the safe side or can I stay for the full 180 days
Completely your choice. Although return airfares tend to be far, far cheaper than two one way flights &, as a Brit, London BKK return is hugely cheaper than booking anything BKK to London.
There is also a possibility to actually extend a Non Imm 'o' visa for an extra number of days without leaving Thailand, through the Thai Immigration Service Offices. I am not sure what the maximum extension supplied is, but for a Tourist visa it is an extra 30 days...