The new Senate sits for the first time tomorrow and the Government will need the support of the five left-wing Greens, the more conservative Family First's Steve Fielding and Independent Nick Xenophon to pass its legislation if the Coalition opposes it.

Opposition Leader Brendan Nelson confirmed yesterday that his party would seek to block several measures among them several budget measures, including tax increases on alcopops and luxury cars, the FuelWatch scheme and the increase in the Medicare surcharge but promised it would not try to block Supply.

''Of course we won't do anything of the sort,'' he said.

Finance Minister Lindsay Tanner accused Dr Nelson of playing politics and warned he was '' playing with fire because if they punch holes in the budget's surplus, the end result is that it is harder for the Reserve Bank to reduce interest rates''.

Recent debate has focused on how the Government will deal with a disparate cross-benches to get its legislation through, but Mr Tanner shifted the focus back to the Coalition yesterday.

Mr Tanner said on Channel 10's Meet the Press, ''Dr Nelson is fond of describing himself and his party as the alternative government. It's about time they start acting like an alternative government. Because minor parties have no responsibility: they will never be in charge of the nation's affairs. They can say and do anything and not be accountable for it, whereas the Opposition has responsibility because they could conceivably be the government. They are the issue. The only reason that the minor parties' votes even matter is because Dr Nelson and the Liberal Party are trying to take an axe to the budget.''

Senator Xenophon, who provides one of those eight votes the Government could need to pass its Bills, said the upper house needed to have a watchdog role. He told ABC-TV's Insiders, ''Australians don't want a rubber-stamp Senate and as we have seen in the past, I think, the Senate can sometimes save governments from themselves. ... I think some of the Liberal Party wish they were saved from WorkChoices with a Senate that could have blocked it.''

Maverick Nationals senator Barnaby Joyce, who has crossed the floor to support Labor in the past, predicted the new Senate would get back ''to what it's supposed to do''.

David McLennan