i'VE OVERSTAYED, WHAT IS GOING TO HAPPEN!?

Firstly, I am aware what I did is wrong and I deserve to be punished.
I've overstayed my visa by quite a few months to help out a friend who was in dire straits.

I was on a working holiday visa first.

I am going to contact immigration Monday and let them know I'm illegal and planning to leave the country on Wednesday as I've booked a flight.

I've booked a flight to Thailand as my mother is meeting me there and has purchased a flight home from Bangkok for me, I'm from Scotland.

dOES THE FACT that I've overstayed in Oz affect me in any other country.

I would appreciate anyones help!

I can answer the original question with accuracy as a former Northern Territory Police offer, where Police in the NT are Immigration Agents and have dealt with this exact situation a number of time.

What will happen is this....
Upon attending an Immigration Official at the airport, any person overstaying their visa will be directed to a room to speak with an immigration officer in greater detail. At that moment, you are in custody, but not under arrest. You are not free to leave, and must answer a large number of questions.
You say you overstayed a working visa. . That's bad for you.
You will most likely be arrested, detained for further interviews which are not brief. Normally you will be held for several hours for interview. The outcome of the interview determines what happens next.
Regardless of which way they go, being they allow you to leave or the deport you, you will have missed your flight.
If deportation is considered or is to be done, expect to be in custody for as much 3 - 5 days.
The other option is just to hide within Australia, wait until they catch you, definitely get arrested, and definitely spend days in custody and definitely get deported.

I am reminded of one individuals case where the man did everything right, except he overstayed his visitor visa by 3 days.

The man was flying from Ayers Rock to Sydney, via Alice Springs. His flight out of Australia was per-booked from Sydney. While packing to leave, he noticed his visitor visa had expired 3 days earlier. He telephoned Immigration to advise them and ask for advice. He was advised to speak with an Immigration Officer at Alice Springs upon his arrival while waiting for the connecting flight to Sydney.
He met with an Immigration Officer at Alice Springs where he was spoken to briefly, then taken into custody and handed over to Police who held him in the Police lock-up for 2 nights. He was interviewed further. He then was put on a plane in custody to Darwin where he was held in custody for a further 3 days before being put on a flight to London from Darwin. He was deported, and refused reentry for 3 years.
He was apart from being deported, sent the bill courtesy of the Australian Government via the British Government - Darwin To London via Singapore about $3000, $175 per night for each night in detention, plus the airfare from Alice Springs to Darwin. The earlier fare he prepaid/prebooked from Alice Springs to Sydney then to London was unrefundable.

I note the original poster says he is flying out tomorrow (Wednesday). Good Luck !!