Evening all.

So ive been saving for a house for a while now. Its always been my goal to have my own home.
3 years ago I meet a girl who had already brought a house about 12months earlier. Me, her and her sister all live in the house, and until the start of this year, I only paid the 'rent'.

My partner gave birth 6 months ago and isnt working at the moment (taking a year off - another 6 months)
We looked at getting a 2nd property, we went into the bank that has her mortgage and with my savings and her equity it would be possible (Based on her being back at work)

before this decisions is made, what is the best thing to do with my savings?

her interest rate was fixed at round 5.5% but that is due to come off. (I see mentions of 5% 2 year mortgages at the moment)
My savings is with rabobank, so my current interest rate is 4.05%

My situation is as follows:
$15k in Savings (inc about $2k in managed funds that havnt been in long)
$14k total in kiwisaver


So for the best return for money, what would be my best approach?
My thought is the best return for money would be for me to move my savings onto her mortgage, taking a principle down a chunk - but not actually 'buy' into the house.
We have talked about putting extra money on the mortgage next year (over and above the required) to try and smash it back a bit.

So in my head, im getting 4% interest saving it, but not being charged 5% on the mortgage.
interest Calculators confuse me a bit, I found a compounding calculator that said:
5% per year on $20k = $1128.16 interest
4.05% per year on 20k = $825.21
So that would mean $302.95 extra paid off the mortgage then if i saved for a year and put it on the mortgage.
However, sorted.org 's calculator says 4.05% on 20k for a year is only $402 in interest.

Now, for kiwisaver.
Because she already owns the house, we are in a position where i dont actually 'need' to use my kiwisaver to buy into the house.
However if I buy into the house, I can only do that once, so im trying to work out the best time to actually do that.
My thought is that kiwisaver would be best used in a few years.
at the moment there is $14k, so apparently the $1k opening and what the government puts in cant be used. so that means about $10 could be used.
The house is also under the value for our area, so I could get the $3k for first home (I know there was talks of changing this to $5k, not sure if thats gone through though)
Im on 4% kiwisaver, If im putting in $40 per week, my employer is putting in $30per week.
to me, thats a 70% per WEEK interest rate. better return than paying the mortgage.

over the next two year, at $70 per week thats $7280 (excludes govts contributions)
over 4 years that $14560
With the orginal $10,000ish on top of that.
More years, more bonus

Would it be better to wait and use it when its at a higher value, or better to put a chunk on the mortgage earlier and then its done.
Thats how I work it out, buy I could be wrong, so im trying to find out.

Flaws and Notes
1) This doesnt account for any relationship issues that may or maynot happen in the future
2) in resent evaluations (estimates) the house is getting close to the first home buyers limit, meaning at this rate, before a couple of years is up, I might not be elligible for the $3k (or $5k)
3) buying a 2nd house with kiwisaver now could mean capital gains on that property with or without capital gains tax
4) If I do buy in now, if we do decide to get a 2nd house, it would be all equity no savings

anyone got any thoughts, what ive done right or wrong.