you may be able to get some additional height but come winter they deflate. Some forms are better than others. Try O. strigil or some of the hardy O. engelmannii/phaeacantha types.

This one got pretty tall here in Massachusetts.

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Hey- If you're in Victoria you should have no problem (given excellent drainage) growing some of the zone 8 Opuntias that can become trees... You might have some trouble with plants toppling due to the lower light levels, but that would take years! I'd look into the plants that Tim suggests as well as things like O. chlorotica: http://www.calflora.net/bloomingplants/pancakepricklypear.html (the form I have comes from an area that has historic lows in the -20F range, I've been too wimpy to try it outside!).

Do you have a 100% sunny spot (6-8 hours + of sun/day)? I'd probably be able to send you some cuttings to try. Tender, half hardy, hardy... The problem for you will not be cold but moisture!