Having a drainage pit and pump inside the house for waste water means you always have dirty water in the pit. This will smell and I would expect (even in Queenslnd) does not meet basic drainage requirements.
Electric pumps are usually an expensive pain in the backside and often prone to failure after a couple of years. I take it this problem only arises after washing machine stuffs up? Do you ever get seepage coming up through the floor after heavy rain etc?
If no then The other way to fix it is raise the machine on a mantle and have made up a drip tray just like used to be put under water tanks when they were up in ceilings with and outlet through a trap so there is a water seal (thus no smell) if there is an overflow the water runs straight into the drain that already takes away the waste water.
Cheers Terry
Ex Plumber.

