If your current property is valued at $270K and you currently have a mortgage of $200k, then your equity in the current property is $70K or 25.9% This means that your LVR is about 74%. So since your current LVR is below 80% you don't pay any LMI (Loan Mortgage Insurance).
If you were looking to buy another property worth approximately $300K you would need to have more than $114K in equity (I say more because you would have to include the extra required for closing costs - including Stamp Duty if required and conveyancing costs in addition to the $114K) to keep your LVR to 80% or below and avoiding LMI. (20% of $570K = $114K).
So if you want to avoid paying LMI you will need an additional $50K or so at least. Even if you went for a LVR of 95% on the new property you would still need $69K plus closing costs, so you are still a bit short. You would also have to pay LMI on the new mortgage and you would be taking on a lot of extra risk, especially if interest rates rise (as they are likely to in 12 to 18 months time), you lost your job or some other emergency came up.
You are better off waiting some time before purchasing a second property, allowing your equity to grow further and for you to save up a bit more. You may want to open up an offset account linked to your loan account (if you haven't already) so that any additional savings you have in the offset account reduces the interest payable on your mortgage.
When you are ready to purchase a second property allow an additional 2-3% in your serviceability calculations on top of the mortgage interest rates you get, so that you don't get into trouble when interest rates do go up, or if you are on a fixed rate, when your fixed period ends and the variable at the time is higher than your fixed rate.