I'd tend to agree with Simon, I like having more less expensive properties than say 1 bigger one. Also means you should have more consistent growth, while one stagnates for a couple of years, the other ones may still increase - therefore not forcing you to postpone your strategy waiting for that 1 IP to start increasing again.
Especially Justadreamer if you're looking at the Adelaide market where you can get cheaper houses anyway for say around $200k.
With your $150k cash I'd get 2 IP's at around $200k each (or even 1 for $250k and one for $150k etc), then put the rest into managed funds (as others have mentioned, I personally think cash in a savings acc. is a total waste, you just end up treading water with inflation & tax)
This way you have at least 2 IP's increasing in value, and therefore decreasing the reliance on equity build up in only 1 IP. You also then have an income from the Managed Funds that you can either roll over and let it increase for the next time you get itchy trigger fingers for another IP, or you could use the interest to help fund any shortfall in cashflow on the IP's.
Cheers
Steve
PS Although if it was me, I'd put some money directly into some good shares I know too, now just managed funds