Proof of onward flight in Solomons

I'm a bit worried about having to proove onward journey when travelling to the Solomons from Brisbane as I don't know how long I want to stay there.

Do i have to have an onward flight?

your visa will be issued based on your outbound flight. they always ask to see your ticket as you go in through immigration. If in doubt, get an open ticket with a return date no more than 3 months ahead. 3 months is the max visa per calendar year for tourists.

Second attempt due to website hiccup.
You could buy a Pacific Blue cheap flight (which can't be changed) back to Brisbane on a date 3 months after your flight in (see 3 month Visa comment above), and you would stand to lose $200 - $250 AUD, and ensure your entry.
You could buy a return Solomon Airlines flight from Brisbane (cheaper than 2 one way flights with them, with the return 3 months later). They will allow you to alter the date of the return flight.
Or if you are planning to go on to Vanuatu, look at Solomon Airlines, Air Vanuatu or Air Pacific for their conditions. You may then need an onward ticket out of Vanuatu, but you may not need to buy it until just before you go there.

I just want to add that not only will Solomons immigration want to see your onward ticket, but check-in staff at the airport where you fly to the Sols from may well check it, too!

I flew from the Solomons to Fiji on Air Pacific, and also found them very flexible about changing the date of my flight free of charge.

Check-in staff at Brisbane airport always check return tickets and passports for flights to Solomon Islands. This includes both Pacific Blue and Solomon Airlines.