Meet Senator Rex Patrick, the man in the middle with a key vote in the tax cuts debate

And if the Morrison tax bonus lands in your bank account next week, you will have witnessed the power he wields, alongside Centre Alliance offsider Stirling Griff and Jacqui Lambie. All eyes are on the troika as the tax cuts are debated on Thursday.

"There's no question we will have influence in this parliament; in the last parliament we exercised our influence responsibly and that's what we'll do here," Senator Patrick says, as he negotiates laboriously with the Coalition on lower gas prices in exchange for his tax vote.

Senator Lambie is more colourful about the pivotal spot she holds, warning the Liberal men not to "crap down her throat" as they did last time she was in the Senate, and to treat her "a helluva lot better".

"Otherwise, mate, if I'm part of that balance of power we're going to have a lot of difficulty getting things through and it's going to cost you a lot of money," Senator Lambie warned on election night.

On Wednesday afternoon, she slapped her invoice on the table, demanding debt relief for Tasmania so it can spend more money on social housing. Senator Lambie earned a cuddle from Liberal man Mathias Cormann at the Senate's first sitting on Monday. She might not get the same after Wednesday's demand.

Senator Patrick, in contrast, has been open to all-comers, spelling out the detail of what he wants - changes that will bring gas down to about $7 a gigajoule, instead of $9, moving to gas as a much better interim energy source than coal en route to renewables.

Asked for the other big issues on which he will exercise his power, Senator Patrick does not look to the glamorous or controversial, but firmly to South Australia. He names an inquiry into the Murray Darling basin plan (that one appears to be already in the bag) and a ban on drilling in the Great Australian Bight.

These prosaic demands seem in character. He starts a conversation with a description of legislation as black, white or grey. Which translates roughly as no way, sure why not, and not so simple.

In the not-so-simple camp is the government plan to outlaw religious discrimination and cement religious freedoms in schools. The details are smoodgy at best on this. The government is briefing backbenchers but refusing to release its draft legislation on discrimination - and has sent the question of LGBTIQ students and teachers in religious schools off to the Australian Law Reform Commission for report.

Senator Patrick's inclination is to keep this issue in the grey area, rather than try to pass prescriptive laws.

"In some sense, the parliament ought to set loose boundaries, perhaps a little bit further than most people would feel completely comfortable with, then let the courts work out the bits in between."

He sets a few scenarios:

"If I'm in a [religious] school as a teacher ... and I say "marriage is between a man and a woman in accordance with our faith, but Australian law allows for same-sex marriage", I think everybody would be comfortable with that.

"But what happens if you say ... "marriage is between a man and a woman and everything else is sin". Is that crossing a line? ...

"Then you ask the question, OK, what happens if a kid now goes out into the playground and confronts an LGBTQI kid and says "you're a sinner, you're a sinner, you're a sinner"."

Or, Israel Folau terms, it would probably be acceptable for him to say what he did in a church, but probably not outside.

"I don't agree with what he said, but I'm neither religious, nor in the LGBTIQ camp, but I respect the rights of both," Senator Patrick says.

This is where he resides; the man in the middle.

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Politically, Senator Patrick says he's literally centrist. He has voted for both major parties and landed firmly in the centre on the ABC's Vote Compass survey, but worked for Liberal David Johnston in opposition - although says that was because of his interest in defence.

Senator Patrick is the son of New Zealand parents, who brought their young family to Australia, Rex aged seven. He left home at 16 to join the navy, eventually becoming a submariner.

Asked what motivated him at that age, he offers no insights but responds in character: "It looked like a good idea at the time".

The early career "taught me about hard work, it taught me about responsibility - on a submarine doing your job correctly is important because you can kill everyone if you don't", he says.

"It taught me about how to get on with people. You live in a very tiny small space for anything up to six to eight weeks at a time ... We had a shower once a week whether we needed it or not. It was a pretty tough, dust-bowl kind of life."

Possibly not as tough as what Senator Lambie says she has metaphorically endured at the hands of Liberal men. But no doubt quite a handy classroom for a career at Parliament House, isolated by moat-like ring roads, at once surrounded by people and alone in the middle.