Penny Whetton and Janet Rice hold hands as they stroll through their impressive garden in suburban Melbourne, chatting about their renovation.

They are a picture of a modern Australian couple, however their marriage is particularly unique.

Ms Rice is a federal Greens senator, Ms Whetton is a renowned climatologist, and they are one of the few same-sex couples in Australia in a legally binding marriage.

"I met Penny as an undergraduate at university" Senator Rice said.

"And then I fell in love, it certainly was love at first sight."

The couple married in 1986 and several years into the marriage, Penny revealed to Janet a deep secret she could not continue to hide.

"It's been with me as long as I could remember," Ms Whetton said.

"I hoped it would go away when I met Janet but it was always there and it worked its way back and I had to address it."

Penny Whetton says she kept her gender identity a secret for years. ( ABC News: Danielle Bonica )

Penny, then living as a man, made the decision to transition and become a woman, fearing it would end their marriage.

"I was afraid for years by even admitting to this that it would lead to the end of the relationship, that's why it took me so long because the relationship was so incredibly important," she said.

Senator Rice recalls the time.

"She shared her gender dysphoria with me and after a period of time we said, 'OK we still love each other and we're going to stay together,'" she said.

"Actually, I remember this moment where we were out dancing and Penny was dressed as Penny and I thought yeah she's pretty attractive and I thought wow I'm still so deeply in love with this person."

Although Ms Whetton is considered a woman in almost every respect, for her to remain legally married she cannot change the sex on her birth certificate.

Senator Rice then said she went through her own transition of acknowledging that her once-conventional heterosexual marriage had in fact become a same-sex marriage.

"There was a shift in my perception of myself from being heterosexual to bisexual but certainly the biggest shift was just how society treated you," she said.

"We had many years of being outward to the world, of being the perfect normal couple and then suddenly we had the experience of what it's like to be in a same-sex relationship."

The pair say relationships like theirs are not reflected in Australian statistics. ( ABC News: Danielle Bonica )

The pair have two adult children.

It is unclear how many couples are in a similar situation to Senator Rice and Ms Whetton, where a same-sex couple is still legally married.

They say there is an under-representation of the LGBTI community in Australian Bureau of Statistics data.

Key dates in SSM postal survey: August 24 — the final day to register with the AEC if you want to take part in the survey

September 12 — survey forms start being sent out

September 25 — all forms are expected to have been sent

October 27 — forms are strongly encouraged to be returned by this date

November 7 — the final deadline to return surveys

November 15 — results are released

According to the ABS, there are at least 46,000 same-sex couples in Australia, up significantly from about 33,000 in 2011.

In 2011 though, 32,300 of the same-sex couples considered themselves to be de facto relationships, while the rest identified their relationships as marriages.

But the ABS does not keep statistics of transgender people in Australia and census data does not include any same-sex couple who do not live in the same home.

Senator Rice is now using her position as a senator to campaign for the legalisation of same-sex marriage.

She hopes that her marriage will no longer be considered a loophole and that other Australian LGBTI people can experience what she describes as the "marital privileges" she and her wife enjoy.

"We know that there are far more than 46,000 same-sex couples in the country but there are people that still don't feel comfortable acknowledging their relationship even to the Australian Bureau of Statistics," Senator Rice said.

"I sort of take for granted that we're married, but it was important for me at the time because I loved Janet and I wanted to spend the rest of my life with her," Ms Whetton said, holding back tears.