Some Tax Questions

Hi everyone,
Just signed up today, i’ll give you my situation, and some questions.

I purchased my first house 1 year ago, that i am currently residing in. (which i have to do for my first home buyers grant) I am living with a mate, who is paying some board. Just under the table so to speak at the moment.

I was talkin to a mate at work who was saying that if i was to put the income on next years tax that i could do a few things: 1 was to say that i am getting a percentage of rent, like say i’m gettting $50 a week, but fill in my tax return saying that i’m getting $20 a week. He was of the viewpoint that this would work for depreciation and that a percentage of my interest being paid from my loan would be able to be claimed. (last year i paid nearly $9000 interest)
Before i moved into my house, i was living with my grandfather, who is still in the house, my mate also said that i could say that i was just solely renting out my house, and say that i was back living at my old address. To get more tax cuts. I realise that this isn’t being entirely honest, and i’m not too clued up on the whole tax thing through investment properties, as i haven’t needed to look into it yet. But do either of these ideas/ways sound worth doing? Too risky? Could it come back to bite me in the bum in the future?
Or would it just be entirely better to just keep my $50 a week rent under the table and not bother about tax deductions.
Any advice on the tax thing would be appreciated, or any things that i could do in my situation?

Thanks