Replacement schemes would be discussed with colleagues after last month's vote to leave the European Union, he said.
Mr Eustice previously said that Welsh farmers would get "as much support" as currently if the UK left.
Under the EU Common Agricultural Policy Wales receives Â£250m a year in direct payments to farmers.
That is in addition to more than Â£500m between 2014-2020 to run a rural development programme.
Mr Eustice  was speaking to BBC Wales at the Royal Welsh Show, in Llanelwedd, Powys.
"What I can guarantee is that...once we leave the EU, we won't be spending that budget on EU membership, and the things that currently we delegate to the EU we will be funding ourselves," he said.
"The point I'm making is that now we have a new government in place, before we can guarantee exactly the same amount of money, that is a discussion we've obviously got to have with government colleagues.
"I can't do it at the moment, but it doesn't mean it won't happen."
He added: "Clearly we need to have the time to have these discussions and to work out what we're going to do and how we're going to do this negotiation, what the time frame is, and then how we're going to fund policies going forward."
Mr Eustice, who campaigned for Leave, said he was clear through the referendum campaign "that when you take back control for a policy and take back control of it on things like agriculture...with that comes a responsibility to get it right and to fund it properly.
"That is the case that I will be making, and I'm confident that will be the position that the government adopts."
Earlier, the Welsh Government's Environment Secretary Lesley Griffiths has said that Brexit was an opportunity "forge a new, distinct path for Wales' farming food and environmental sectors".
But Welsh Conservative leader Andrew RT Davies warned ministers in Wales lack the capacity to make Wales-only agriculture policies after Brexit.
Mr Eustice said: "What we've got to work out is a UK wide framework for an agriculture policy.
"There will be issues such as international trade that will obviously be done by the UK...
"What I'm keen to do is have as much discretion as possible for individual devolved administrations to pursue what they think is right for their own agriculture.
"I see a situation where the Welsh administration will have far more power than it has now, but will still need to have some kind of UK wide framework to prevent anti-competitive policies getting in place."
He said he wouldn't want to have "distorting subsidies in place that affected one area more than another".
He said "broadly" Westminster would be in charge of agriculture cash, with the Welsh Government given its share, but with more "discretion" for Wales to spend the money in the way it likes.
UK farming minister George Eustice has said he cannot guarantee that future agricultural support programmes will be as generous as current EU subsidies.