Do I apply for British Citizenship?

Hi All
I’m wondering if anyone can help, I’ve emailed so many places and am still confused. I am wanting to Marry my partner in England next year, at a registry office as my family will be together at this time. I have read through stuff regarding this but am a little confused, my situation is as follows.
I was born in Australia in 1978.
My mother is a British citizen that has lived in Australia since 1969. She has a British passport.
Can I just apply for a marriage visitor visa or do I need to apply to become a British citizen? I’ve read that if you’re eligible for citizenship you can’t apply for a marriage visa and I’m confused if I actually am or not as I was born before 1983. If someone could help I’d be most appreciative.

Read the following:
https://www.gov.uk/apply-citizenship-br ... efore-1983
Section "https://www.gov.uk/apply-citizenship-br ... efore-1983"
I'm not an expert on this but it would appear if your mother only was a British citizen at the time of your birth then you need to apply though the cost is just about £100. As a British citizen you don't need a visa to come to the UK.

You need to apply to register as British on form UKM and submit all the supporting documents, cost is £80. Once approved and you attend your ceremony and receive your certificate, then you can apply for a passport.

Ok, so I will have to apply to become a British citizen and not just a marriage visa, so when I return to Australia will I be a dual citizen? Thank you for replying.