Lots of good advice from some wise heads in this thread.
I'll add my own amateurish note.
Steveo's list is gold. Personal debt, then mortgage, then invest for retirement.
I'd skip any extra super contributions whilst you smash the mortgage. Relationship stress whilst living frugally to achieve a common goal is much less stressful than being a slave to a mortgage during tough times. It can be relationship building and very positively reinforcing to team up on that debt and work together. If you both went at it full bore, then in only 3 years you could be complete home owners right around the time you want to be starting that family. I can imagine worse goals. Your combined salary is great at present. Achieve as much as you can whilst it lasts.
Use the time whilst paying off the mortgage to learn about investing. Read, and read some more. Come up with a mock asset allocation, then track it during those years to see how you feel as it goes up and down, and try and get a feel for an appropriate asset allocation that suits both you and your partner. You may miss out on a rise in the stock market, or a fall, or a static period, but you'll be banking a guaranteed 5% on the mortgage, and be avoiding rushing into something you aren't yet comfortable with.
On another note, you are the main bread winner at present, but many couples go through periods where the main bread winner switches. It can be useful to be able to allocate earnings from investments to the lowest income earner, for tax efficiency purposes. When you buy in personal names, then that person must take the income and there is less flexibility. A Discretionary (Family) Trust in Australia is a way to jointly own investments, and you can then alter allocations from earnings to anyone in that trust. I'd recommend you read around this option whilst destroying that mortgage, and if it's something that may suit your situation, then when you start building your taxable investment fund of index shares you can do so in this structure.
I'll be interested to see which way you head, but certainly you are starting from a great position and simply getting on a forum like this and realising there is a lot to learn, is the first and most important step. Good luck.
Murdoch