Best treatment I found over the years for fungus gnats, both preventative and curative, is the combination of BTi and yellow sticky traps.
The BTi is a bacteria
bacillus thuringiensis israelensis that kills the larvae. It's very safe and very effective. Gnatrol is a US brand name of this.
The yellow sticky traps are placed horizontally at medium level. I just have mine laying on the tops of the pots.
These kill the adults. These traps are cheap on Ebay & AliExpress, not so much at bunnings.
The bugaboo is Australia stupidly hasn't approved BTi for this use so it's kinda hard to get exactly what you need.
Some guy on Ebay was importing from Alibaba and selling and I was buying from him but he must have got shut down.
I contacted some suppliers from Alibaba and it would cost about $100US to get 4kg here... shipping being half that cost.
I've considered buying some, perhaps there could be enough folks here to to a group buy.
Wait a second... I just checked ebay again and someone in Oz is selling actual Gnatrol now. I think it's that same guy actually. Awesome! I just bought some until I can maybe make a much cheaper bulk buy from china.
Find many great new & used options and get the best deals for GNATROL BTI Kill Fungus Gnats Larvae 2 OZ (56 grams). 10-40 Applications. at the best online prices at eBay!
www.ebay.com.au